Burris XTRIII are supposed to be factory set at 20MOA down to account for a 20MOA base. Sonit should have been close to zeroed.
The 3.3-18×50 comes with 35MIL of total elevation adjustment. You said you got 39 MIL of travel? Were you dialing while looking through the scope with your rifle secured to watch the reticle move full travel? Could the screws to the turret not be quite tight and allowing the turret to skip adjustment? If it is skipping, 39MIL (390 clicks) could have been 20MIL which would be about right if you were not watching movement.
What rings? Not all are created equal. If your ring caps are too tight, it could be binding up the elevation erector not allowing it to move freely, causing skips too. Try backing the ring caps to 15in/lbs (make sure it is INCH pounds, not FOOT pounds...it has happened and been discussed on here before), and again, watch reticle movement.