This shouldn't be surprising given that even orthography and names were often not standardized consistently for most of history.
For example, Adolf Hitler's grandfather was called Hiedler, not Hitler and the spelling change was the result of his father's name, Alois Hitler, being changed later in his life after first being recorded as Aloys Schicklgruber (the family name being that of his mother rather than father as the fatherhood was apparently initially contested).
Or for orthography you just need to look at any historical text pre-19th century or so and you'll find plenty of oddities that often change regionally or even between writers in the same region.
Now expand this to the time scale and area of the Roman Empire/Republic and it's amazing most of it was somehow coherent over time. Actually as far as I recall, the "subtractive" style was only used consistently in the Middle Ages. Another odd variant I've seen is "IIX" instead of "VIII". And let's not talk about how larger numbers were represented or shenanigans like the "long I" instead of "II".
That was a quick escalation to Hitler - just goes to prove Godwin's law! :)
I didn't realize the subtractive style really dates to the middle ages, but that certainly seems consistent with the coins - I checked a bunch more and none seem to use it.
As far as I know the subtractive style was in use in Rome but the purely additive one was more widely used until the Middle Ages. There are also examples where multiplicatives are used but the only thing I can find on Wikipedia is an example from the Middle Ages: "XIII. M. V. C. III. XX. XIII" (13573 = 13x1000 + 5x100 + 3x20 + 13) and a medieval one using superscript: XV ^C XIX (15x100 + 10 + 9)
As for the Hitler one: he just happens to be a widely known person with a family name that changed not so long ago. Plus I actually first learned this from someone joking about whether he'd have been as successful if his father had never changed his name from Schicklgruber. Fun fact: "Hitler" was officially supposed to be pronounced with a long "i" (ee) like Hiedler but apparently Hitler didn't like this pronunciation and suppressed it once in power.
