In my opinion:
Although that bullet does kinda-sorta look like a "cleaner" bullet, it isn't.
Actually, it is a cast lead bullet whose moldcasting-sprue did not get cut off.
In the photo below, you can see the hole in the mold at the base of the bullet-making cavity, for pouring molten lead into the mold. The mold's filling-hole produces what is called the casting sprue on the base of the bullet. Notice that a small moldseam is clearly visible on the side of this bullet's sprue.
When the newly cast bullet is removed from the mold, the casting sprue is supposed to be cut off the bullet. It looks like in this case that didn't happen.
I should mention, "unfinished" cast bullets like this one have been found on civil war battlefields, because some soldiers had to make their own bullets. I myself have dug some "unfinished" castings like this one.
Notice that a small moldseam is visible on the sprue.