You decided to visit Thailand for whatever reason I don't want to know. (It's not my business.) Assuming you're in from outside of Asia, you'll land at Suvarnabhumi Airport. The problem? The locals don't pronounce it that way. All the names and signs here are romanized using the Royal Thai General System of Transcription (RTGS). The big problem with this is that it is light years away of what Thais actually pronounce. For example:
*'a' as in father
Sri - Devanagari transliteration
Racha - RTGS
As you can see, even words from the same source still use different romanization system. The best of all is a word of Pali-Sanskrit origin can also use both systems to romanize. In the same word. Gott hilf mir.
A lot of words from Pali-Sanskrit entered the Thai language some time before 1500's. (Can't even find a source as to how it got into Thai and what it was used for.)
Boring bits (that you should read)
Some people prefer words of Pali-Sanskrit origin to be romanized using Devanagari transliteration. I should note that Devanagari transliteration is lossless whereas RTGS is lossy. It means that transliteration via the Devanagari system can be transliterate back and forth without mutating the characters whereas RTGS transliteration can't be transliterate back to Thai unless you already know how it's spelled in Thai.
Hence all the problems. RTGS is trying to represent how Thai words are spelled but it does not represent how it's pronounced.
Summary in case you still don't get it
Thai uses RTGS to romanize but the problem is that you can't convert it back to Thai and it does not represent how a word is pronounced by Thais. However, words of Pali-Sanskrit origin can sometimes be romanized using Devanagari transliteration, which again is not even close to what Thais pronounce.
There's another romanization system called "whatever I damn well please use to romanize."