Why does portupgrade sometime fail?

[Rudy]

I ran a
  portupgrade -r glib

and I get this problem with some packages:

===>  Installing for liboil-0.3.14
===>   liboil-0.3.14 depends on executable: pkg-config - found
===>   Generating temporary packing list
===>  Checking if devel/liboil already installed
===>   An older version of devel/liboil is already installed (liboil-0.3.12)
       You may wish to ``make deinstall'' and install this port again
       by ``make reinstall'' to upgrade it properly.
       If you really wish to overwrite the old port of devel/liboil
       without deleting it first, set the variable "FORCE_PKG_REGISTER"
       in your environment or the "make install" command line.

Why doesn't portupgrade just uninstall and upgrade 'liboil'?

I manually went into devel/liboil and ran:
   make deinstall install

Is there a portupgrade flag I am missing?  And I don't want my pkg_info to end up with both versions:
  liboil-0.3.12
  liboil-0.3.14

Thanks,
Rudy