Differing calling conventions can point to combination of hand crafted object system in C with some custom code generator with some high level input that produces machine code directly without C in between. When you generate machine code that does not directly interface with system libraries it is often useful to ignore platform ABI calling conventions and make up your own.
Perhaps they use some kind of right-to-left fastcall convention. Or maybe they are just unconventional, putting "this" at the end of the parameter list, hence ending up in different registers or the stack depending on the number of arguments?
