A couple 0.75" groups sound like normal statistical variation to me. Between rifle, ammo, atmospheric conditions, muzzle velocity, and shooter, a 0.5MOA rifle will sometimes print groups that are 0.75, and other times it'll shoot 0.2-3.
If you want to get a good indication of what your rifle will reliably do, print a 10-shot group. You should never see a 3-shot group exceed this size, and rarely a 5-shot group, assuming mechanical consistency.
Falling rain or snow will not measurably affect the POI of a supersonic bullet, unless it's coming down so heavy you can't see what you're shooting at.