I think a lot about possible explanations for this...
Maybe the different groups of named varieties are just expressions of dominant gene groups, in that scenario there could be several truly different varieties within the Etnas, Vertes, Sicilian Reds etc. that have the same macro features but minor differences due to their unique recessive gene pairs. Fig seedlings are all unique, but areas with smaller gene pools should have less variation in wild populations.
Epigenetics/mutations could be responsible for some differences, figs have been cultivated by cuttings for a long, long time and being in different conditions/selected by growers could be responsible for the differences. I think in the case of epigenetics, two different named varieties that have subtle differences would become more and more alike over time when grown side by side (forgetting their different pasts). Mutations would be more permanent, but seem unlikely to be the cause of multiple differences.
Growing conditions change how a variety expresses itself... I have seen lots of variation from different trees with the same mother due to minor individual circumstances: damage, growth structure/pruning, soil variation, weed pressure etc. So making fair comparisons is hard, maybe impossible when you add in other known and unknown factors like tree age, mycorrhizal and endogenous fungi, pests and disease, sun exposure, different mothers, and so on.
I have a very similar situation where I can't tell Tatnall Red and Sal's Corleone plants apart by growth or leaves, but Tatnall has been much more precocious... Can't remember eating a Sal's C, actually, my original plant was in a bad spot for a number of years. But other people have reported that Sal's C is precocious, and while Tatnall usually looks just like Steve's BT and SC usually looks just like that Aldo's, I have seen pictures of Sal's C with the same features as TR and SBT. So I started some side by side last year, but down deep I think that either could outperform the other by chance or favor.
The reason I like to lump named varieties into groups is that it makes selecting what to grow easier... If I like a variety enough to have lots of trees I might get other varieties of the same group as well, hoping for small differences like ripening time, color, growth habit etc. If I have had a bad experience with a variety I try to avoid others from that group. I also think it is much safer because of the reasons listed in #3, things like fruit size, color, taste, vigor, etc. can change from one year to the next. I think it would take a longterm, meticulous trial to really prove anything, and even then some people would probably still disagree, there will always be another example of a type to trial also.