Player performance is mainly dictated by a hidden factor. This factor is constantly changed by events. It's like the morale bar but the game don't display it to you (if it did, the game would take no skill to play and the form history would be pointless).

Players on slump (6 or below form continously) will eventually get past it and turn normal again.
Players on fire (8 or above form continously) also can't maintain it forever.

Consistant training from the facility improves form but this is not fool-proof.

It takes skill to keep good performing players on the pitch while sticking players under slump on the bench.
The player form history help you to determine this.