Faceting, both today, but especially when it was done manually, is not always as precise as you may think. I have found lots of new Czech beads where something went wrong while faceting. One side gets no facets, the other side gets cut too much, etc.
Without really being able to tell from the picture, I would think that this may be what happened: a faceting mishap, leading to a unique bead.

In general, these beads are firs molded, and then facetted. Pretty much the same way that modern day 'fire polish' beads are made: from a molded round, oval or pear shaped bead, made into the desired faceted shape.