Turns out that there is already an APAR for this, IX85298.

At 12:43 PM 1/14/99 -0000, you wrote:
>We have recently noticed that after upgrading to 3.1.2.1 on AIX and NT
>platforms, more versions of changed files are kept than the parameters in
>the ACTIVE copygroups stipulate.
>
>For example:
>
>If policies are set so that:
>
>3 copies are kept if the data exists
>1 copy is kept if the data is deleted
>
>If a file then changes and is backed up 5 times, 5 copies are kept! Even if
>the data is then deleted from the client and another backup is performed. We
>have also run expire inventory with no change.
>
>Has anyone else out there found this?
>
>The time based expiration for these versions still seems to work ok.
>