When we start the game we have a building untill the top that scolls.
Then when we broke the glass the scroll of the backs darker buildings stops but the bottom ones continue to scroll.
Again when we entered on a right side building taller (untill the top) the building is placed on screen on those back buildings that weren't scrolling.
And on this version it also seems that some of those gfxs above the buildings (not the sprites you can crash with them) also seem to have to some kind of parallax.
The other version (the unoficial one by MrSid doesn't have any of this, it's all things horizontal scrolling at the same time/speed).
http://www.youtube.com/watch?v=7I1zqkyIYC4
Anyinformation you know?