The -w and -W options behave differently and produce different output when 
the stdout is a tty tty than when it is not, e.i. 

$ man -W info
/var/catman/local/cat3/info.3.gz
/usr/local/man/man3/info.3

$ man -W info | cat
/usr/local/man/man3/info.3

$ man -W info | more
/usr/local/man/man3/info.3


I haven't checked the source code, but I imagine that there is a test 
for 'isatty' early in the man program to determine whether or not to 
utilize the preformatted 'catman' pages.  This test should always be true 
when the -W or -w options are in effect, otherwise it is easy to miss the 
source of corrupted man pages.

Incidentally, the 'man' manpage mentions the ability redirect the output 
of -W but does not say anything about the fact that the output will be 
*different* than if you just let it go to a terminal.  I don't think the 
man page is incorrect for this oversight; I think the behavior is broken.