Why conf-split prime?

[Stephen Froehlich]

Its not an issue for me as I have 10 users and don't expect queues larger
than 2 or three messages (i.e. performance is not an issue).  I wanted qmail
for the security, though I am more than pleased to have the scalability and
speed.  However, why should this number be prime, why not have 12 or 16
directories?

Just curious

Thanks