Why are the numbers so different?

It's because they are guesses. The only way to find out how much you should eat is trial and error.

+/- 10% accuracy at best.

Seems odd that MFP thinks your TDEE is 2590 for sedentary though

What does http://www.myfitnesspal.com/tools/bmr-calculator say ?

Here on MFP it says if I want to lose 2 lbs a week I need to eat 1590 calories, and then any exercise I do. However, on a TDEE calculator it says I need 2391 to maintain my weight, which means that I would only be allowed 1391 a day. This is a difference of 199 calories a day! In a month this would be the difference of almost 2 lbs! Why is this so different? Which should I follow?

Different calculators can use different formulas. In the grand scheme of things it really isn't a huge varation. It probably wouldn't be a 2lbs difference in the end. Like someone else above said, these are all just estimates. Keep in mind this whole game is just an estimate, regardless of how meticulously you log your food, there will be variations. We cannot be exact outside of a lab. Pick a number, try it for a few weeks (more than 2) and adjust from there.
Personally, I prefer to start high then move low. I would like to keep my calorie intake at the highest I can and still lose weight at a reasonable pace. Plus, then you are just following MFP since you are here. Pick whichever you want and feel you can manage. Or average the two.

v

what is your activity level on MFP and on the TDEE calculator...?

Couch potato! I want to keep my exercise separate.

Keep in mind if you choose to do it that way, it is not actually TDEE since TDEE is total daily energy expenditure, which would include your exercise. I only say this because in the future if you ask more questions and mention TDEE the answers will be based on the assumption your exercise is included. It causes a lot of confusion around here.

the things you are mentioning that you do not like that MFP doesnt account for is why you do not figure in that you are a couch potato though. you atleast go with somewhat active which most people are at the least. Than you add your actual exercise in after words and eat half of the total value of that. The calorie calc is going off true honesty and not us rigging it. But as others have mentioned you really have to give it trial and error and you seem your are losing so things sound like you are going in the right direction. And if you ever do stop get a food scale and see how that works.

It's because they are guesses. The only way to find out how much you should eat is trial and error.

How come all the good answers all already taken?:sad:

I said a month actually. 199 Calories times 31 days is almost 2 pounds a MONTH. I don't believe anyone should really lose more than 2 lbs a week anyways.

I have been doing fine losing weight on MFP, I am just not satisfied with the way I feel. I will give it more time and see how it goes before I switch to anything else. I find MFP difficult because it doesn't count calories for things like hours of shopping, or other not everyday, but mundane activities.

actually it does count shopping for hours, its called walking at leisurely pace. I have the same dilemma when it comes to what to actually listen to: do I want to use mfp's guided settings or use my tdee which includes my daily activities and exercise. I found that when I first started and used mfp's settings I have lost weight just fine but now that I lost more and am more active than when I started, I hit a plateu so I have mine set up at tdee and losing 20% of the tdee. it gives me more calories than what mfp gives me but I am losing again. MFP is designed so that you are initially eating at a sedentary level and you have to input your exercise and activity calories (yes that includes shopping or cleaning) and then eat those calories back. TDEE is your total daily energy expenditure (basic metabolic plus extra calories expended for the day. if you use this method then you don't have to eat back your exercise calories burned because the amount of exercise is already taken into account for the tdee. either method is going to put you at a deficit but you have to pick one and be honest about how active or inactive you are. definitely count your calories eaten correctly too. there are many different formulas out there for figuring out your bmr and tdee. harris benedic and katch mcadle are common ones but there are others. you will find a that with each formula/method you will come up with a different number. don't let this confuse you too much. remember these are gueses/ estimators. every person is different. what works for you may not work for me. they all indeed work. the trick is to pick one and stick with it. trust me on this: I have looked into a lot of them. I found some good sites. I personally have my calories set to tdee minus 20%. so far so good as long as I do the exercise that I said I do (moderate level 3-5 days of exercise). don't get too caught up in the numbers. just pick one and follow through like the other responders have stated. try it out for 2-3 weeks and see if it is working for you. if not then tweak it a bit by 1-200 calories. the best advice that I found lately is that you need to eat more to lose... this is so true for me anyways. I hope this helps you. Remember this is a learning process with every week you are doing it. Good luck.:drinker:

Personally, I think they are all just estimates and it's up to you to see what works best for you.trial and error.