Mouse scroll and trackpad behaving differently
Shift/scrollwheel is horizontal scroll, which will only work, of course, if there is somewhere to scroll to horizontally.

I've never used a trackpad, but I presume horizontal scroll on a trackpad can be quite specifically indicated by horizontal movement - it can't, of course, on a mouse-wheel.

As noted in comments by timothymh, SteerMouse ($20) can override this behaviour &amp; set Page Up/Page Down to the shift/scroll action.