Question about XP vs VX Event Movements using Scripts

[Heretic86]

I'll take it that the two engines handle Event Movements completely differently.

Im trying to borrow some of the features Modern Algebra put in Zeraibs Caterpillar and effectively port them over to XP.  Feature I got working (mostly) and trying to convert has to do with Actor Passability.  Mine sucks.  His Rox.  When I do a movement in XP, some other actors can still pass thru the caterpillar, as if they've left the spot and the next actor hasnt stepped into that spot yet.  Doesnt seem to be a problem at all in VX.

Is this just because of the difference in the engines, or something more specific with the script?