Can anyone share with me how does the airport tax be calculated or defined?

I used to fly with AirAsia from Johor Bharu but I don’t understand why Senai airport tax is higher than LCCT airport tax for flying to the same destination (either domestic or international flight). I also like to find out if we have to pay higher airport tax for the further routes from the same airport. And why do some countries impose higher airport tax while some airports are tax free?