It may well be because it is set up as a US Open course and therefore a par 70; this hole may be one that is changed from par 4 to par 3 for this tournament only, hence why it is par 3 on WGT.
Yardages for holes and their relative par's are, as Boxall says, down to playing length, and because this hole is so much downhill it is considered to be a par 3. A bit like the 900yd par 3 at Legends Golf & Safari I posted about somwhere, possibly in the Courses section.