I'm wondering what "airport capacity" actually means. The numbers that are talked about in the newspapers and online articles do not make sense to me. Here is a great example:
https://en.wikipedia.org/wiki/Phuket...tional_Airport

In 2015, Phuket International Airport 12.5 million passengers.
The capacity of the airport is 6.5 million. They are building a new terminal which should open later this year, which will bring the airport capacity up to 12.5 million passengers.

How is this possible? How can they get double the amount of passengers through the airport over what their official capacity is? If the answer is, "oh, that's because XYZ" then why isn't the current capacity 12.5 million and with the expansion the new capacity will be 20 million (or whatever)?