Zen Cart is a fork of osCommerce. This apparently happened because some of the developers wanted to do things a little differently to others. This happens with Open Source products. It's not neccessarily a bad thing, just confusing. Sometimes the changes that are implemented in a forked product end up ported back into the original product and so both benefit from each others work.
The key is understanding, or finding out, why the fork occured and whether the changes in the fork are what you want.