In many freerolls, folks enter and then forget, or leave, or something. Was at its extreme at party poker
where 60% of the people would not show up after signing up.
The longer the lead time between start of sign up, and starting time, the worse it is.
Then there are the players who win big early, and sit out for a while thinking the maniacs will weed themselves out, and they can come back 40 minutes into the contest and play a more civilized game.
There are some who sit looking like they are sitting out, but are right there waiting for the big hand. They will catch you trying to exploit the sit out behind you. They are rare though.
For the most part if you have sit outs behind you, they can work to your advantage. Easy to buy the button if no one is there to call you.