Semantics. Pypy's RPython dialect is "a restricted subset of Python that is amenable to static analysis", to quote the PyPy website.

RPython and Pypy are different things.

RPython is a restricted subset of python indeed, but its purpose is to be a toolkit for implementing virtual machines. It is not and does not aim to be a general-purpose programming environment.

PyPy is a JITed Python runtime implemented in RPython.

The relation between PyPy and RPython is more or less the relation between CPython and C.