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.