If you create a tag and a bookmark on the same changeset, then use the History View to examine the tags and bookmarks, the tag is shown coloured like a bookmark in the Description column and is incorrectly described as a bookmark in the Type column of the Labels panel.
If you remove the bookmark, then the tag is shown correctly.
The following will reproduce the problem in version 0.9.16 of MacHG:
hg init testrepo
echo some text > file1.txt
hg add file1.txt
hg ci -m "Initial revision"
hg tag MYTAG
hg bookmark -r MYTAG MYBOOKMARK