Cannons have the tendency to deal block damage deeper than they hit, or already deal damage in front of the spot they hit (I suspect that since it's hard to double check the latter).
The damage numbers are at the correct spot though.

It's really tricky to reproduce this behavior with a single weapon group, if at all.
You can get this behavior though with a cannon that has multiple groups in it. In my example I used 8 weapon groups and usually about 2 to 4 of them dealt damage deeper or in front of the spot that got hit.