Your powder is too slow to burn up in that short barrel. Common snubby problem. If you have a chronograph you discover the velocity spread is high, too. I've seen up to 25% ES.
At the other extreme, N310 will burn clean as a whistle and give much lower MV spread, but you will lose velocity because you can't safely use enough of it to go as fast. You could try N320 or N330, too. Peak velocity at safe pressure will go up, but so will fouling and velocity spread.
I don't have numbers for VV, but all powders have some bulk density tolerance. Lee's VMD's are taken from samples they bought, but due to the lot variations they are almost never exactly what you have. On this side of the pond only Western Powders publishes average bulk densities and gives tolerances, which vary by powder quite a bit and don't match Lee's numbers well in some cases. (see below). So keep on with the cut and try approach to landing a charge weight.