It’s presumably a mistake. Sometimes PGs system has a problem or I suppose someone has neglected to update the discounted fare inventory. Or, it could just mean that the flight is very full on that date and only full fare inventory remains available.

I would give a call to PG reservations on Skype, or at least wait before booking. If the direct flight continues to be expensive, you could consider a connection in BKK, although that is less desirable of course.