This is the official (and correct) explanation.
Vacuum varies with a maximum of atmospheric pressure (valve closed) or 29.92 Hg", a minimum in low performance engines (equipped with a 1 or 2 bbl. carburetor) of about 3 Hg" down to about 1.5 Hg" in high performance engines (4 bbl.). EFI can be even lower since vacuum is not needed to draw fuel into the engine.
According to David Vizard, in a race engine with intake and exhaust lengths tuned for a harmonic resonance to assist cylinder fill, the returning exhaust pulse is stronger than the piston's draw.