One thing I'll never understand about Scheme/Lisp is why they need so many versions (of the standard, and of different compilers).
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.
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?
See also: Common Lisp, which has a similar problem.