Why is "volatileQualifiedExpr + volatileQualifiedExpr" not necessarily UB in C but in C++?

When I today read the C Standard, it says about side effects

Accessing a volatile object, modifying an object, modifying a file, or calling a function that does any of those operations are all side effects

and the C++ Standard says

Accessing an object designated by a volatile glvalue (3.10), modifying an object, calling a library I/O function, or calling a function that does any of those operations are all side effects

Hence because both forbid unsequenced side effects to occur on the same scalar object, C allows the following, but C++ makes it undefined behavior

int a = 0;
volatile int *pa = &a;

int b = *pa + *pa;

Am I reading the specifications correctly? And what is the reason for the discrepancy, if so?