Set mc._y within scrollpane?

I think I’m missing something here…

I’m trying to use the scrollpane component for the first time now, and I’m trying to attach a number of mc’s inside a for loop.
After attaching I’d like to position them vertically, but when I set the _y property they don’t ‘stay within the scrollpane’ somehow but continue outside the bounding box :h:

What’s the trick here??

Cheers.