Actually, I'd think this is more of a "geometry" thing than a "physics" thing. Simply put, when a cylinder head is shaved (or a cylinder block is "decked"), it is measured in thousandths of an inch (or millimeter, if you're living in the metric world). Now, this may not seem like much, only talking about paper thickness. HOWEVER, the shorter distance between the drive (crank) toothed belt pulley and driven (camshaft) toothed belt pulley center lines has changed enough that the timing belt tensioner will pull the belt in tighter. This changes the angle of the cam pulley in relation to the crank pulley at top dead center...i.e. the crank is "straight up" and the cam is off by a couple of degrees.
This is why hi-po overhead cam engines frequently use the adjustable cam pulleys, because the head shaving increases compression which is good for performance, and then you can adjust the camshaft back to correct top dead center alignment, or some guys I believe actually advance the cam timing a bit.
Hope I haven't confused you, it kinda confuses me, actually!
Regards,
Shea