Lead temperature and mold temp play a big a part also. Get the mold hotter and the bullets will drop heavier.
And I've noticed that mold material also can do difference. Iron,Brass,Aluminum... Brass and Iron gets hotter quicker and also keeps temperature better than Aluminum...
I have a PID controller controlling my RCBS pot. It keeps the temp within about 10 degrees. I have a 92 gr .257 mold that will drop 91.7-91.9 gr. I can get most of my molds to drop within .5 gr but they are all steel. If they don't I just seperate in .5 gr. Then shoot like weight like Nick said.