C++ diverged from C in 1982-1983, and that’s a long time in computer years. But, there are many C libraries with C++ compatibility, including the C standard library itself, and a steady stream of programs are ported across from C to C++. Many C programmers only know or use the features that are compatible with C++.
They are defined by different ISO standards from separate committees. Even when they define compatible features, it is often defined in different terms.
Referring to C/C++ is about as valid as referring to Italian/Spanish. You should be careful to whom and when you use such a term. But it’s true that there is diffusion of ideas in both directions, and the similarities are more than coincidence.