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The three metals that make up the bullet alloy of casting are, Tin, Lead, and Antimony. If an amount of those pure metals are put into a melting pot and the temperature is raised, the Tin will melt first, then as the temperature continues to rise, the lead will melt and lastly the antimony will melt (but usually at a temperature (600+ degrees Celsius)too high for some/most home equipment to reach). When the metals have melted they combine as liquids into a homogeneous mixture.
The high melting temperate of the antimony is the reason that bullet casters do not try to alloy pure antimony into their casting alloys...they buy or obtain antimony already alloyed with lead/tin because the melting temperature of the combined metals is much lower than pure antimony.
As an alloy of Lead, Tin, and Antimony cool, the Antimony returns to the solid state first, resulting in the "mushy" appearance of the cooling bullet alloy.
Not a scientifically correct interpretation of what happens, but a pragmatic interpretation of what happens. But, for almost all bullet casters, the answer to your question would be: They just do.
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