The spawns you get are totally independent from the ingame difficulty. The difficulty just adjusts the damage you do to enemies and how much they do to you. What you detect is just the lack of variety within the Draugr level lists. Normally the encounter difficulty you set for a reference gets calculated by the fLeveledActorMult for the set difficulty. But all this settings are just as good as the level list they are based on. With level 45 you almost reached the end of the list. The flags that are set for the level list plays also a role. Do they allow spawns with a lower level than the player ones? If I remember right, the level lists for Draugr doesn't have "Calculate from all levels <= player's level" flag.
Maybe you should create a new lvlnpc based on a new custom level list. The vanilla game offers a lot of sublists you can use to cover a higher level range and more variety. This way you can control everything you like. It also ensures compatibility with mods altering these lists.