hmmm ... best way I can explain it a sine wave on an ocilliscope.
You've got that wave action of the barrel. Depending on what part of the frequency the bullet departs the barrel, you get the bullet "tossed out" at a slightly different point of barrel "tip up."
OK, I suck at trying to describe it
... anybody else?
Too, using a heavier bullet, all things being equal, will cause a bit more recoil which could tend to toss the bullets a bit higher.