A Tibetan syllable that ends in two consonants is very often ending in a grammatical marker rather than in part of a word, and the second of the two is normally sa.
The second suffix slot, the yang jug, takes only sa in the modern orthography, with the classical da drag as the other historical occupant. What sits there is typically a case particle or a verb ending fused onto the syllable it belongs to: the genitive is the most frequent, and several of the other cases and a number of verbal forms behave the same way.
For a reader this is a useful rule of thumb. Two finals means look for grammar. The word to be looked up in a dictionary is what remains when the second suffix has been taken off, and the second suffix itself is telling the reader what the word is doing in the sentence rather than what it means.
It is also one of the places where written Tibetan and spoken Tibetan diverge visibly. The second suffix has left the modern spoken language almost entirely, surviving as an effect on the vowel and the tone where it survives at all, so a syllable written with six letters may be spoken with three sounds and a pitch. The information is not lost; it has been recoded, and the writing is the record of where it used to sit.