Not an expert on Thai linguistics, but a phonology enthusiast who's skimmed the phonology section of the Wikipedia article for the Thai language, and I thought I'd provide my understanding of why this confusion occurs. (It's similar to a common issue in romanisation of Standard Chinese.)
The consonant group in question is the "velar plosives", so-called because they are produced by the sudden release of air blocked behind the tongue and the velum. In English, this includes the k and g sounds.
There are two relevant modifiers that can be applied to consonant sounds: voice and aspiration. Voice is the vibration of the vocal chords. If you touch your throat, you will feel the difference between s and z. Aspiration is the "h-iness" of some sounds, where a short burst of breath is produced alongside the consonant.
In common English dialects, the k sound is unvoiced and aspirated, whereas the g is voiced and unaspirated. The two properties go hand in hand, so either can be used to distinguish the consonants. (0)
In Thai (as I understand it), there is an unvoiced and aspirated sound, and an unvoiced and unaspirated sound. So whether these two sounds are interpreted as k/g or two varieties of k (kh/k) is ultimately arbitrary.
Pinyin, the most common romanisation scheme for Standard Chinese, went down the "k/g" route. It sounds like RTGS goes down the "kh/k" route instead.
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(0) In actual speech, this apparently clean distinction is much more blurry for a variety of reasons, but it's a good enough approximation for this purpose. For the interested you can Google "fortis and lenis".