From memory the "thin" and "thick" silver finish classes recognised that there were two competing but roughly equally popular IAg chemistries in the industry , each with their own process max-min deposit thicknesses and one typically thicker than the other. 

Clearly it didn't make sense for an IPC spec to have a "barn door" range from minimum "thin" to maximum "thick", hence two ranges defined. I don't recall one being "better" than the other because of the thickness