It is because of the alloys chosen for each part of LBT molds, the venting, and perhaps above all, the expensive to make sprue plate design. The key is to force the base to solidify last and have a puddle of molten lead to fill the shrinking/hardening lead.
Anyone with access to a milling machine and optical magnifiers can check this out by milling bullets completely to chips with a razor sharp mill cutter. As you come to them the voids are visable. There are none in bullets CAST AS DIRECTED in LBT molds. Nose pour bullets are full of them and cannot be avoided.