Student or Learner
It is often said that low vowels are produced with open jaw. One can produce low vowels with close jaw. The trick is to cup the tongue. In other words, one can produce vowels just with tongue action alone: cupping, arching, bunching, etc. Different languages use different settings for sake of articulatory ease. Just because one particular setting is easy for speakers of a particular dialect; it does not mean that such a setting is easy for everyone else. This has to do with how neuromuscular system is trained by their first language/dialect, thereby becoming 'natural'.
Well, I am not an expert on phonetics, and have never claimed to be. And, I do not see the relevance of your post to the original question, which is why I asked about it. So, could you please explain for the non-experts how that answers eijpoo's question?