top moving down, middle moving up

It all depends on their PC. If a top house has a couple bad pledge classes in a row they move down, if they have good ones then they move up.

like Alpha Phi is generally a top house but their babies this year weren't great so it causes them to move down a little.
Kappa got a great pledge class this year so it causes them to move up.

Also its what they are like around the frats and how they interact with one another.
A sorority could be awesome at the frats and well liked, which causes them to move up or vise versa.
A sorority could have an awesome sisterhood or a horrible sisterhood.