[Zope] threaded?

I'm new to Zope. My understanding is that Zope and Medusa are threaded (as opposed to multi-process). If that is so, why does my Zope installation keep forking itself?

pstree -puc 6631 python(6631,thepler)---python(6632)---python(6636)-+-python(6637) |-python(6638) |-python(6639) `-python(6640)

-todd