According to Wikipedia it is a way of showing respect... ok so far. But then it adds “The higher the hands, the more respect is shown”. Look back at the pic and tell me who is showing respect to whom! Shameful.
You are correct about the higher hand position denoting greater respect to the person being greeted.
According to the link below there are five distinct presentations of the Khmer sampeah greeting which can be loosely described as caste based. Follow the link to see the flash video of Khmer examples.
http://vogel.km-kh.org/gallery/movie/2010/579-sampeah-etiquette-in-khmer