ASCII is a 7-bit encoding. That’s it.

Honestly I’m not even convinced it’s entirely fair to call code page 437 a superset of ASCII, with how it repurposes the control codes 0x01–0x1F and 0x7F. (Superset of printable ASCII, definitely. It just feels not quite right to call it a superset of ASCII as a whole, though it is generally considered so.)

(And I can’t find any references to code page 437 being called “IBM PC ASCII”.)