I'm confused?.......If you shot over a deer's back, you didn't shoot 3-4" higher than you should have. More like a foot or so. Are you really aiming that high when you shoot?

I think you might have your numbers a bit underestimated......if not YIKES.

But if you ARE aiming where you should be (1/3 to 1/2 of the way up the chest just behind the front leg) and still shooting over you might be "Peeking" in the excitement, overestimating the range, or something changed in the way your bow shoots.
Have you shot your bow lately? I shoot mine EVERYDAY even if its just a few arrows to stay in practice and make sure there are no surprises.
Check your bow and where you are aiming AND PICK A SPOT on that deer's chest and shoot at it......not just the whole deer.