This would explain it thought...
Drift@10mph 90° .2" 0.7" 1.5" 2.8" 4.4" 6.5" 9" 12" 15.5" 19.6"
1.5" is exactly what I measured on my target too.
Either way, I will go back on a non-windy day and shoot a few more. I made a shot two years ago from 189 yards away and I dropped him like a fly.