[pygy_]

This is probably negligible, but the hammer is not perfectly rigid. The tension in the stem at the time the hammer escapes and subsequent oscillations could also play a role.

[smj-edison]

That's a fair point, but I don't think that would make a split between timbre and amplitude.

[jcims]

This little morsel was coagulating in my mind as I was reading all of these comments. No idea what, if any, effect there would be, but it is a variable.