If you activate two pistons at the same time, one piston will extend (see pictures), after a few attempts, a different piston will extend. This occurs not only when you switch them on directly without repeaters, but also with repeaters, on either one tick, two ticks ore three ticks delay.

For example:

1st try: Piston 1 will extend, 2nd try: Piston 1 will extend, 3rd try: Piston 2 will extend (not always the same order of pistons that are extending)

In my opinion redstone, as a logic system, should always have the same delay/update order. Otherwise redstone contraptions, will misbehave.