Keep and Untagged seem to compound, is that intentional?

**Describe the bug**
First of all, not sure if this is a bug. But let me describe a scenario.

When you have a repository with 50 tags all linked to a single manifest (so 50 manifests) and you run purge on this 1 repository with "--keep 10 --untagged" and for simplicity lets say you set "--ago 0d".

Then purge would first untag 40 manifests, leaving you with 10 still tagged.
And after that, it would remove 30 of the 40 untagged manifests.

The end result being:
10 tags
20 manifests

**To Reproduce**
See description

**Expected behavior**
With the example above in mind, I somehow expected to end up with 10 tags and 10 manifests.
I'm just wondering if how it currently works is intentional or a byproduct of adding (the keep) functionality where this is a (happy) accident.

**Any relevant environment information**
- OS: Windows
- Version (acr-cli) 0.19