Let us not forget that phase diagrams in reference books are equilibrium
phase diagrams.  In real world solidification rarely occurs under ideal
equilibrium conditions.  The problem is that non-equilibrium
solidification can produce undesirable microstructures.  That could be
the reason for slightly higher tin content in the "practical eutectic"
alloy vs. the "theoretical" eutectic alloy.  Tin-lead eutectic (61.9%
Sn) while solidified under non-equilibrium conditions will produce a
microstructure containing lead-rich primary crystals having mechanical
properties inferior to these of fully eutectic microstructure.  Slight
enrichment of the composition in tin (63% Sn) prevents formation of
lead-rich primary crystals and results in a superior fully eutectic
microstructure.  (Reference: "A SCIENTIFIC GUIDE TO SURFACE MOUNT
TECHNOLOGY" by Colin Lea).

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