For me prices for plane tickets vary depending on the season. Usually, summer seasons are more expensive in beach places such as Krabi, Phuket, Koh Samui etc. as compared to city places like Bangkok. Also, it can be due to the fact that there are usually less passengers to phuket as compared to bangkok since Bangkok is often considered as a gateway of not only to Thailand but also to South East Asia. So, perhaps airlines try to jack it up in order to get a bit more sales.