Why is the indefinite article before RTS 'an' instead of 'a'?

edit

RTS stands for Real Time Strategy and is a game genre. However if you want to specify a game of this genre you say 'an RTS game' instead of 'a RTS game'. Why? As I am not a native speaker, nor a linguist, I am unsure if I am corerct in thinking that 'an' should mean that the first letter of the word is a vowel. In this case, however, this rule doesn't follow. An example:

https://en.wikipedia.org/wiki/Real-ti...

1st sentence of 2nd paragraph.