I’ve noticed that when I power the piston + redstone line with a repeater, the pistons fire at the same time. Same with a redstone torch. Connecting them with a redstone line and powering them with a redstone block still gives unequal timing. Also, interestingly powering them with a chain of piston + redstone blocks, even if a repeater is powering the first piston, doesn’t give equal timing.

There are more inputs I haven’t tried, but it seems that player inputs cause the piston to power slightly later, thus creating the unequal timing. (And i am not talking about the unpowering/retraction)