Lead expands as it is heated. It continues to expand once molten and the temperature continues to climb.
Very very hot lead in the pot and then into the mold will start to cool.
If the lead solidifies before it has time to draw sufficient material from the sprue pool the bullet could have an occlusion in it. V(Here you will see it called a "bubble". Bullets cast with adequate sprue pools do not get bubbles. I had that argument with a guy here that swears EVERY bullet has as a bubble, I invited him over and we cut open 3 balls he made and 3 balls I made - 2 of his had a bubble - none of mine did.
@hrt4me and I did some casting last week and he witnessed how and why bullets get bubbles. We had a fun day!
When the lead in the mold solidifies on the outer shell first, the hotter the lead was, the more it will shrink.
We are talking thousandths of an inch and hundredth of a grain here. It is not enough to have any effect on a black powder bullet with "normal" loading and shooting.
If you are talking custom long range shooting - it probably could. That would be less than .001% of black powder shooters shooting out past 500 yards and farther.