Wear on the die as well as differences in flow of the plastic can affect the size and shape of the lettering on the outer layer. I believe the actual plastic in the letters comes from the liquid plastic on the layer below that squeezes through the outline of the molded letters during the secondary molding stage.

In any case, it isn't an extremely precise molding operation and can result in slight differences in the lettering. Also notice that the two examples you posted in your scan show the edge of a sheet of clear plastic over the chip. This can distort the view in some cases where the edge overlaps the lettering (as on the letter "i").

I don't think the designer specified different fonts when the tooling die was made, at least based on what I've seen. Whether the differences in the final chip justify assigning added catalog numbers is up to the author producing the catalog.