The class B/C distinction is based on where the transmitter site is located. Southern California (indeed, all of California except the extreme north) is in the FCC's "Zone I-A," where class A, B and B1 stations are allowed. Nevada is in "Zone II," where class A, C3, C2, C1, C0 and C stations are allowed.
KHYZ has never applied for a Nevada transmitter site (except for the Las Vegas booster CP), so it's always been in B territory. However, it did request - and receive - a waiver allowing it to exceed normal class B power limits to allow for better coverage of "white areas" along the I-15 corridor where there's no other local radio reception. Its present licensed facility (8.4-kw/551 m) is a super-power B; its construction permit is for a standard 50-kw/150 m B at a location closer to Las Vegas, where it would trade some of its I-15 coverage (less important now that everyone's listening to their iPods or satellite radio or whatever along that long drive) for a signal that can be boostered into Vegas.