As stated in the title, pistons check whether they are able to extend immediately after being powered, rather than waiting until the block event phase of the tick. What this means is that, if a piston that would cause a slime structure to exceed the push limit is powered before the piston pushing the slime, the slime-pushing-piston will still count it in its push limit check because it has not extended yet.

In the attached image, you can see a circuit that demonstrates this.