Look at the WGT description of OAK #9:
This very difficult, blind, uphill driving hole has a severe ditch left
and severe pot bunkers right. This pivotal hole, which plays as a par 5
for the members, played as a long par 4 for the U.S. Open. The huge,
severely undulating green also serves as the practice putting green
As a par 5, it certainly plays a lot easier (only 477 yds = 430 m from the tips), and the HCP numbering will be based on that situation (pros don't have course ratings).