Maybe because those white teachers were ordained in the Thai tradition? White monks ordained in the Burmese tradition may be called "Ashin", for example Ashin Ottama. Although that isn't always so, for example in the case of Bhikkhu Pesala.pilgrim wrote:OK, why do western sangha call their white teachers by the Thai word Ajahn? Why not use the original Pali Acariya?
Then there are monks like Venerable Amarantho who is technically a samanera and has been for over ten years. But he prefers being called by his monastic name, Amarantho, without any prefixes or suffixes.
Ajahn Sujato once said that sometimes his fellow Australians call him "mate".
If you're not sure how to address a monk, "Venerable" seems a pretty safe choice.