Can anyone help me understand this?

Unless you ate 7000 calories over your maintenance you didn't gain 2.2lb overnight. It is just water weight and probably leftover food in your stomach since you ate more than usual. It will be gone as soon as you get back to normal, no need to reduce your calories by 600 today, just get back on track and it will come off again.

More sodium = water weight. Plus poop. Don't worry about it.

Depends on what you ate. If it had a lot of salt, your body compensates by holding onto water. Plus if you haven't pooped, that weight is still in there.

600 calories is 0.172 lbs. 3500 calories is 1 lb. Keep that in mind.

If I go for sushi, I use too much soy sauce and am pretty much guaranteed to go 1-2 lbs the next day. Takes 2-3 days to flush out the extra sodium and water and I'm back down. Doesn't mean I'm going to stop going for sushi though. And it doesn't mean I gained 2 lbs of fat.

Don't cut back that much in one day, spread it out over 2-3 days. Also try not to focus on what the scale says today, but focus on the trend of your weight. Use a trending app to keep track of it.

It's frustrating, I know. About a month ago, I registered a close-to-five pound gain after getting significantly more exercise than usual (and eating a bit more, since I try to eat back 50% of my exercise calories). The thing is, hanging out in these forums in the preceding weeks had helped. Because once I recognized that to put on 1 lb, I'd have to be eating 3,500 calories above maintenance... well, there was absolutely no way that I'd ingested 17,500 over maintenance overnight. Therefore it had to be water weight.

Not going to lie. I was still upset seeing that 5-lb jump. Imagine that you're driving somewhere on the highway and you hear over your radio that, due to construction, a stretch of the route you're driving on has been reduced from four lanes of traffic to two and it's bumper-to-bumper. Nothing you can do about it. No way to get off the road now. And, sure enough, a minute or two later, you're stuck in an unmoving line of cars. It's frustrating. It's annoying. But you also know why it's happening and that you aren't going to be stuck in that line forever. You'll get there. It's just going to take a little longer.

The water weight will pass. Unless you ate more than 7,000 calories over maintenance yesterday, and it doesn't sound like you did, it'll drop soon.

You need to get out of the mind frame of weighing yourself after an evening out.
For gods sake go out, enjoy the food, enjoy the company then get back on plan the next day. Weigh once a week, once a month whatever but stop holding yourself to ransom with the weighing scales! You understand about Cico and needing a deficit of 3500 to lose 1 lb of weight? Well think about it then, how could 600 calls put up 2.2lbs? Weigh yourself tomorrow and you will see your down already. Plus alcohol, alcohol,always causes you to carry excess fluid especially the next day.

It'll be gone in a couple of days!