There are some explanations to this. There has been a graphic bug in AH1 and 2 for a long time. If you shoot at Gvs at long distance, sometimes the kicked up dust from a missed shot will show up INFRONT of the vehicle, even if your shot is long. This makes it very hard to judge if your shots are long or short. If you are using HE-rounds, even a near miss will damage the target vehicle, especially if it is "soft", like a M16 or M3 (making HE the preferable round to engage those targets with).
So possibly you were just missing the target, but still damaging it from the near miss.
I have, however, also seen AP-shots glance of an M3 or M16. There are also accounts of AP-shots going THROUGH an M3 or M16 without doing much damage, i.e. passing through a soft part, just punching a small hole, and getting out on the other side (kicking up dust). With the current level of rudimentary accuracy regarding the effects of different ammuntions and armors on vehicles I strongly doubt that this would be modeled, however.
Litjan