Could be a descendant, like his great great grandson or something.
Even Lance Henrikson isn't clear on who he was supposed to be at the end of Alien 3. In the script and credits he's listed as Bishop II, which clearly implies that he's supposed to be another droid just like the first Bishop. However, in the film itself he identifies himself as a human, and this is supported by the fact that he bleeds red blood after getting conked on the head.
What's dumber is, if all the droids are supposed to "look like Weyland" then what the hell was Ash? Yutani?