Anytime you return an SV* from an Inline C function, it creates an XS wrapper that calls sv_2mortal() on your return value.
I don't think this has anything to do with Inline::C. I think it's something that xsubpp does in creating the C file from the XS file. If I'm not mistaken, you'll get exactly the same thing in a normal XS environment (ie even of you're not using Inline::C).