I think the game's explanation as to why it only sets to 17 instead of the 19 is the fact it's been damaged - i.e. "Warped Headband of Intellect".
Kinda how they justified removing damage reduction from Imps in the tutorial by calling them "lesser imps".
That wasn't actually a Larian idea. There are lesser imps in the AL module "Escape from Elturgard" and other lesser devils in the various T1 modules in that series.