Ruin Explorer
Legend
I don't find it ridiculous. I can easily picture how the one might differ from the other, and I can also think of a bunch of reasons why they aren't the same proficiency from a game design standpoint. An artist, like a calligrapher, would have a kit that included his favored tools - the sort of nibs and ink he prefers, the type and color of wax he likes best, whatever parchment suits his needs and tastes - you get the picture, an artists build a kit based on personal preference. Putting a forgery kit together is entirely different, you pick nibs and inks common to certain kinds of document production and in a wide enough range to cover most of the likely needs, you have a variety of paper common to different sorts of usage, etc etc. The rationale behind the construction and use of the two is entirely different.
I'm not going to argue that the skill set needed to use the two doesn't overlap, because it does, but I think there's enough separation that the fact that they are currently different in D&D isn't obviously ridiculous.
I think you have an idea of calligraphers/scribes and their social status and role that is at very hard odds with their actual social status and role in medieval/renaissance society. Most of what they did was either hastily writing stuff down for people, or carefully copying stuff - noticing the copying stuff. A calligrapher/scribe cannot possibly get away with just having the stuff he "favours". He's not that high-status. His job involves copying. His job involves making replica texts. It's forgery in all but name (and criminal status).
In a fantasy society it may be that calligraphers are some sort of exalted beings carried around on palanquins or whatever, who only do the work they want to, the way they want to, but I mean, I don't think that's intended to be the default.