Unless I'm mistaken, I think it happens because the
head of the description is evaluated twice, thus incrementing the
counter twice---at least, that's the only explanation for how the number
is incremented by 2 each time. Further, this issue seems to persist
whether we're using macros or writing \incrementcounter directly.
The solution is to either make sure that the head of
descriptions is evaluated only once, or document that text placed as the
head of descriptions is meant to be evaluated twice, if there's a good
reason for doing so. Of course, this silly example of incrementing
counters can be subdued by placing the \incrementcounter directly in
the after
option when customizing descriptions. But it demonstrates the issue.
I've tried to make the examples given be as varied as
possible, demonstrating the different ways in which descriptions
interact with the counter.
The second bug is that descriptions, or at least
that's my intuitive expectation, shouldn't interfere with the bullets of
items. I expect that the bullet stays where it is, and the description
is placed next to it. But what happens is that the bullet is shifted to
the right, making it overlap with the description's head, which isn't
pretty.