Bio means life or living!! 

Cell isn't mechanical or not completely mechanical. Notice that he has living tissue. (example) During the battle with Gohan, he is blasted by an enormous Kamehameha Wave. as a result he loses a few limbs, and starts to BLEED! MACHINES DON'T BLEED!! This is proof that he is more than a simple Jinzoningen! Also notice that #16-#20 don't regenerate! while cell is the only Jinzoningen that does, besides Coola if he even counts!