Hammer butt under felt question

[Kevin S. Riggs]

Hi folks!
 Can anyone on the list tell me why the under felt on a vertical
hammer butt is two pieces, one thicker and one thinner(and more dense)?
I have searched the archives to no avail but may be using the wrong
keywords.

Thanks,

Kevin Riggs
Associate in NC

Always trying to understand things not meant to be understood(by me)!