it's a very interesting subject
Your and your friends' observing seems to be a strong anecdotal bit of evidence for this....
First, I think castnblast is right - they do not tumble - either type of bullet is designed to penetrate in a straight line and mushroom. UNLESS it hits heavy bone, then all bets are off - it may tumble; it may not.
My SWAG would be, that IF this is a valid actual phenomenon, as it seems to possibly be, then it is likely due to extra hydrostatic shock (wound channel) cause by the rear of the bullet being wider and causing a wider "trail" for the hydrostatic shock, than a boattail that more easily "slips through" the flesh, with a smaller "tail" or "trail" of hydrostatic shock. But again, just a SWAG....
Boattail bullets are not more accurate. They have a bit longer PBR, due to having a better BC. They also may leave a smidge less copper in your barrel.