Strike Tone not Yielding same beatspeed

[Michael Jorgensen]

Hello,
     Steinway B,  F2-A flat 2 minor third tested using C5 as strike tone
yields a significantly faster beat speed than listening to the beatspeed
by playing the minor third by itself.  This happens with single strings
or when testing the whole unison.
     What is the reason???  F2 is the lowest plain string with weird
inharmonicity perhaps?  How can the strike tone clearly yield a
different beatspeed?  Perhaps Virgil Smith has an answer??
-Mike Jorgensen