With that bullet and velocity a 3mph(90 degree) wind will put you off 10" or so at 800 and a 5mph wind will put you off around 17". Wind may have been 3mph or so at you, you may have had a stronger crosswind a little further out. If it grouped just fine and you made hits when compensating for the off target impacts and it grouped fine at 100 & 500 yards. Then everything points to the wind call, sounds as if the rifle set up is good to go.