I studied American Sign Language for 3 years in University (more of a drinking club with credits than anything else). We were taught that a lot of signed language is like this- that signs can be invented as needed and agreed upon and that's just a normal part of what makes Signed languages cool.
As a result, it was expected that if we traveled a few hundred km in any direction, the local signs might be dramatically different than the ones we're used to, and so we'd better learn to interpret context and ask what specific words meant.
Signed Languages are different. It's hard for people who don't talk a Signed Language to understand because of the Blub Paradox.
The word "Blub Paradox" in itself is a good example of what Deaf people do if they encounter concepts they need to communicate. They make up words. Signed Languages are very productive in making up words.
So it is not just an idiom, slang or jargon, it is more than that.
I'd suppose it's no different than the wide use of local colloquialisms, abbreviations, and pronunciation differences in spoken language region by region. An American in Australia has to intuitively pick up a lot of new vocabulary in order to understand what's going on in addition to the obvious parsing of the accent.
As a result, it was expected that if we traveled a few hundred km in any direction, the local signs might be dramatically different than the ones we're used to, and so we'd better learn to interpret context and ask what specific words meant.