Too many calories for additional exercise

its called exercise calories ..some eat them some dont ...both ways seem to work though but i think the kinds of foods you choose to eat with the exercise calories plays a big part...personally i stick to a set calorie amount regardless unless I feel starved and then i add in fruits and veggies

Did you burn 2100 calories through exercise? or is that what MFP is suggesting in total after you entered your exercise?

MFP sets you up with a caloric deficit to meet your weight loss goal. i.e. 2 pounds per week is a 1000 calories deficit. If you exercise you increase that deficit which means you are no longer set up to lose your goal amount per week so MFP adds them back to keep your deficit at your goal amount.

But keep in mind that you have to back out the calories that you would have burned at rest during those 2 hours as they are already accounted for in your daily expenditure and goal calories. Most people burn 1 to 1.5 cals per minute at rest so in 120 min (2 hours) you would have burned about 120 calories anyway that are being double counted. When exercising for longer than 1/2 and hour these double counted calories should be removed by the user when entering the info into MFP. so if you burned 900 calories in those 2 hours you should only put in 780.

Eating the number of calories that are adjusted after exercise calories have been added will help maintain your weight, but not lose or lose as much if you stick with your planned calorie intake.

This is incorrect information, your deficit to lose is calculated in you base amount. This only adds what you burn through exercise leaving your goal caloric deficit intact, your goal of 1 or 2 pound loss per week does not change, and this is the only way for it not to change.

The only way to maintain your weight would be if you were set up to maintain your weight then ate your exercise calories. If you have a goal to lose 2 pounds per week you are supposed to eat the calories burned to keep your deficit to lose 2 pounds per week, if not a few things can happen:
Your deficit may be to large and your metabolism could slow down making it harder to lose weight
You increase your deficit so you may lose weight faster
your metabolism may slow down and you may not have any energy as food is fuel and you would be burning way more than you put in.

Only one of the above is good and after a while of this the other 2 will slowly come into play.

But keep in mind that you have to back out the calories that you would have burned at rest during those 2 hours as they are already accounted for in your daily expenditure and goal calories. Most people burn 1 to 1.5 cals per minute at rest so in 120 min (2 hours) you would have burned about 120 calories anyway that are being double counted. When exercising for longer than 1/2 and hour these double counted calories should be removed by the user when entering the info into MFP.

That is fascinating and I had not heard that. I was wondering why if I ate back my exercise calories I kept gaining. Now, I'm only 15 lbs. from my absolute goal weight, so I know it'll be hard to lose and much easier to gain, but that is great information.
Thanks for that!