This is a bug related to teams and dead players. When a player is dead, the game used to still recognize that they were still in the game. Instead, testfor only relays the amount of players alive in the game.

To reproduce:
Set up a clock activating a testfor @a with two players on the server.
Have one person die while the other watches the circuit.
It will decrease by one while they are on the respawn screen.

Hope this helps you fix the bug.