> How many C implementations/compilers are there?
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?
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.
Some of them include their own features on top of the standard. Some of them focus on specific use cases and/or optimize for specific performance characteristics. Some of them are toy projects.
How many C implementations/compilers are there? Even Python (which has a massive and often ill-defined spec) has multiple implementations.