You could say that all of human experience is arbitrary and all ideas are made up...
The difference between arbitrary and non-arbitrary in this respect is whether someone who speaks the language and has a general relevant technical background would understand what the name means, in principle, or not. If the developer makes up something arbitrary themselves, then they would understand it, and so would one or two people who review the code or interact with it. Anyone else coming onto the scene would _not_ understand what that's about. You would end up with a glossary of bespoke terms not known to anybody else in the world. Now just think what were to happen if each repository of code had its own glossary of this kind. (shivers)
