I am guessing it is because they can get more going out than coming back on a one-way. Why, I have absolutely no idea.

Oops. didn't scroll down. It's because the more expensive flight starts earlier.

Supply & demand ?

PG have four (4) dailies between BKK and PNH.

PG have at least six (6) different fare buckets, from lowest price/most restrictive to most expensive/least restrictive, on this route:

Web Saver, Web Delight, Web Flexi, Web Freedom, Business Blue Saver and Business Blue Ribbon

There is limited inventory available in each fare bucket, as the lower-priced fare buckets sell out you are forced into a higher-priced fare bucket.

It’s a bit like fresh fruit, or any perishable commodity, at the supermarket; if the apples which are on sale for 13 baht each are sold out you may want to buy apples which cost 15 baht, assuming you want/need apples that day?

As far as I can determine there is no variability in fare direction on this route with PG, that is to say, other than currency exchange rates, the price to fly PNH-BKK-PNH appears to be the same as BKK-PNH-BKK.