That's fair, until you get into GCC extensions. I guess the difference is that the Scheme "core" is typically smaller and the amount of extensions is typically larger.
See also: Common Lisp, which has a similar problem.
See also: Common Lisp, which has a similar problem.
I can compile same source code with pretty much any C compiler. I might be wrong, but the lisp compilers are all "slightly different". I guess scheme ones can be the same? But are they really?