ད་རུང་། ད་དུང་།

Evidently da rung and da dung (adverbs) “still” or “yet” are synonymous but the question is how come? One feels that da rung is more natural and makes sense also etymologically. Perhaps it can be best etymologised as “even now/then.” Could it be that graphic similarity of rung and dung led to frequent mistakes in the orthography that da dung came to be accepted as a corrected form? Both occur in OTDO but I am not sure of the use of da rung there whereas the use of da dung seems to be clear.