It's probably just what they call it.
I have had people try to explain to me how they are different, but when you remove all their BS, it is the same thing.
The thing everybody else called a "hard disk," for instance, was long referred to by IBMers as either a "Winchester disk" (see https://en.wikipedia.org/wiki/History_of_IBM_magnetic_disk_d... for the etymology on that one) or a "fixed disk."