Airlines evaluate any additional allowance you might be entitled to on the day of flying at check in.
Indeed. My most recent CE booking said I could take 3 bags, but on the day I suspect I will only be allowed 2 since I am currently gold but will be a temporarily embarrassed silver member at the time (a lot of travel ended up so late this year that I didn't regain gold at the end of my status year, and it'll take a couple more trips to get there).
Not a problem in this case, since there are limits to how much even I am willing to buy and carry home from a German Christmas market.