The eleventh of the seventy-two is given the ending -yah in the standard lists; the seventeenth, with the same three-letter root, is given -el. That is difficult to account for on any stated rule.
The received explanation is theological. In the Kabbalistic reading, -el refers the name to the side of judgement and severity, and -yah to the side of mercy, so the choice of suffix places the angel on one or other of the outer pillars of the Tree of Life. Applied to a pair of identical roots, this means the same divine syllable stands on both pillars at once, in two places, according to nothing in the syllable itself.
There are two ways to take that.
One is that the position is doing the work, as it does everywhere else in this system: the eleventh falls in the octad of Chokmah and the seventeenth in the octad of Binah, and the suffix follows from the sephirah rather than the sephirah from the suffix. That is consistent, and it is probably right.
The other is that the suffixes were fixed early on phonetic or scribal grounds and the pillar interpretation was applied later to whatever distribution had already settled. That is also consistent with the evidence and would mean the theological reading is a gloss.
The sources do not decide between them, and no source states a generative rule. What can be said is that the suffixes are not random, they are stable across the tradition, and lists agree on which name takes which, and that no primary text explains them.
An unexplained but stable feature is worth more attention than an explained one. This is where the system's construction is most visible and least discussed.