I need some help please...

I thought BKR was calculated on what our BKR was when a game starts with someone, but it seems like it is actually calculated based on what our BKR is when the game actually finishes????

It doesnt make sense, since we are told during our in game play what the expected BKR change will be if we win or lose or tie, so why is it actually calculated based on when the game finally ends?

So, for example, if I want to play only people with a similar BKR to myself, but the game lasts for a year, i might end up playing against someone who is either far superior or far inferior to me, and my rating will reflect that?????