I speak very little Thai but the issue is I think, that while some foreigners think their Thai is too a high standard it is not spoken the way Thais speak.
I suspect that it is for a number of reasons, tonal, the fact that in the North for example Lana is used more than National Thai and people in different areas have there own accent and slang similar perhaps to what occurs in the UK.
We have a friend who claims to speak Thai well but my wife and one our friends cannot understand him and tell him to speak English to them as both speak fluent English.