they are a popular opinion, taken by a vote, of how good a team was thought to be at that time.
It may not be perfect (it isn't) but it's the best you can do in a sport with this many teams who don't all play each other and have major scheduling disparities...until after the season when all is said and done.
When you take the rankings for what they are (a popular opinion) it's easy to understand that you can only go by what it was at the time the game was played, because the result of the game will alter the popular opinion.
How can anyone ever claim to have beaten the #1 team, if that team drops to #5 "because" you beat them.