Scheme (as in the R5RS spec and earlier) is a very minimal language specification. This means that all practical Scheme implementation add features not present in the specification. In some areas the Scheme implementors agree - and in others: not so much. Racket has many features not covered by R6RS/R7RS. However due to the module and macro system that enables you create languages (that compiles to Racket modules), it is possible for Racket to provide a language compatible with Scheme. That is: Racket includes an implementation of Scheme.