Say this happened in only one factory and the other molds in other factories were blocked correctly for the M.U.S.C.L.E. toyline.
That's assuming there were multiple factories and multiple duplicate molds.
I've done some research in the past trying to find a book or anything about Bandai. From reading their Web site, it appears that they only have one factory. Though I could certainly be wrong.
Also, I believe jkaris has said the molds are extremely expensive... in the thousands of dollars. It is very possible that there were only one to two molds. (We can guess that there were at least two because of the multi-part figures that come with the attachements on either side -- although they could technically have come from the same mold, I suppose, as there are two identical sculpts per mold. Perhaps the extra pieces are attached to the main sculpt differently on the two identical figure sculpts on either side of the main family mold for some reason.)
In any case, though, you may be right. Hell, I'm not even sure the Plastic theory is what happened.
Why are you being so damned persistent, anyway? DID YOU FIND A COLORED SUPER RARE!