ambiguous in pkg_add -ui

[Jan Stary]

I upgraded my current/amd64 and now I am upgrading the packages
with pkg_add -ui; psutils gets upgraded as a dependency of a2ps:

Ambiguous: choose package for psutils-1.17p3-a4
 a       0: <None>
         1: psutils-1.17p3
         2: psutils-1.17p3-a4

I just answer '2' and everything goes on smoothly,
but why is this an ambiguity? Is this expected?
In fact, psutils-1.17p3-a4->psutils-1.17p3-a4

        Jan