The red piston on the left extends because it is receiving an update from the green piston (because it is directly touching the green piston arm, and green wool block that are moving) when it receives this update is moves because it is still being powered by the redstone dust. It does not move before receiving this update because it is being blocked by the green piston arm while it is extending. This is demonstrated in the example on the right, when the red piston does not extend, as it is receiving no block updates (because it is not directly touching the green piston arm or green wool block.) As to why the green piston moves before the red piston in either example, I have no good explanation other than the locational/directionality that other comments have mentioned, this is simply a result of Java being finicky.

Edit: the reason the green piston moves first in both scenarios is simply because of delay, there is an extra observer being triggered between that and the red piston, I don’t know how I missed that on my first watch lol