Why So Much Variation In Riga Free City Schillings 1563-80?

I've been collecting schillings from the Riga Free City period for a while now, but haven't found a good answer as to why there were so many variations. For instance, three examples from 1566 are shown here.

The first has legends MONETA NOVA ARG / MONETA NOVA ARGEN. The second has CIVITAIS RIGEN (more typical, with the city name on the obverse) / MONETA NOVA ARGE. The third CIVITATIS RIGEN / MONETA NO ARG. In addition to the legend differences, the separators between words also change. Across the series, the style of the keys changes from a plain to a fancy style, the position of the cross above the keys shifts (and sometimes is a fleur instead of a cross), etc.

Any ideas on why there is so much variation? One thought that someone had, at least with respect to the more than a dozen legend separators, is that they may be related to various mint masters, but I don't know for sure.

Any thoughts would be welcome. Thanks!