PVS by my understanding will only ever over-count visible polys.

It is essentially a set of all polygons that are visible from any point inside a fixed volume, but the camera only exists at a single point inside that volume so there will probably be some polys that the camera has no LOS (though I suspect these would still be 'rendered') to and a bunch that are out of the view frustum which will not be rendered.

edit: To observe this you can also load any HL1 engine game, run `r_speeds 1` in the console, then it will show you how many world polys are currently being drawn in the corner of your screen, which is probably the count referenced by John Romero here.