Players are replaced when you buy. (Usually by what looks like a better player )
Some days individual players change without me buying.
On 'New players on the Scout list" days sometimes there's a wholesale change and sometimes its partial.

There's probably a slight similarity to the TM in that there is a shared pool of players (greater than the number you see) and what you see can vary slightly at the start of each day with a (unspecified) shuffle on Scout list days.

But I don't stare at it much (because usually if I'm looking for something specific its not there) so essentially I just made all this up.