The description of sp_inact in shadow(5) and usermod(8) does not match the
behavior of Linux-PAM.  In the pam_unix module, sp_inact is used in
conjunction with sp_max to check whether or not a user's account should be
treated as expired.  The documentation, however, indicates that sp_inact is
used in conjunction with sp_expire which is not the case.  Knowing what
that field does by source inspection I suppose I can see how what is in the
man page *might* be interpreted as the real behavior, but it is a stretch.
It is worth noting that Solaris (and other SVR4-based systems) use sp_inact
for something completely different, but that's really a separate issue.