A canted reticle won't cause it, but tilting the rifle different ways from shot to shot will.
As will parallax and wind, as centershot pointed out. The effect of wind-drift doesn't just double at twice the range: Instead it basically quadruples.
I'm pretty sure I can rule out the rifle tilt and the wind was less than 5 mph on both days, so that is out too. I'm assuming parallax issue, which we tried to adjust that prior to the 2nd day of shooting, but it didn't seem to help.
