The first four lines of the Tibetan alphabet are a grid. Four places of articulation, four letters each, running unaspirated, aspirated, voiced, nasal, without exception. The fifth line is a Tibetan extension: three affricates built on the palatal shapes, plus wa. And then the arrangement runs out of pattern.
The sixth line holds zha, za, the small a and ya. It is not a place of articulation and it is not a manner series. It gathers two fricatives that the grid has no slot for, a weak consonant whose real work is orthographic rather than phonetic, and a semivowel. The seventh line, ra la sha sa, is similar: two of its members are the letters that carry almost the whole weight of the stacking system, and the other two are fricatives. The eighth holds ha and the great a and has only two members at all.
This is what an inherited system looks like after it has been fitted to a language it was not made for. The parts that transferred cleanly stayed in formation. The parts that did not were arranged into lines of four because four is the shape the recitation had already established, and the lines are containers rather than categories.
It is worth saying plainly, because a learner who expects the last three lines to mean something the way the first four do will look for a pattern that is not there.