Oddly enough those are something different. Each monster has a spawn rating and a level. Those level milestones are the different spawn ratings a monster may have. The spawn rating is decided by a few tags just above the code you have displayed. This way you can specify the power of a unit in terms of level and spawn rating when either summoning or spawning. As far as I know, the only time a monster uses anything but the defined spawn rating you see here is when a player spawns units from a lair and quests. Quests can use any spawn rating. Lairs will use the min and max spawn ratings defined here.
I would like some per level bonuses to at least elementals so that they can do lots or little elemental damage based on level. It also would not hurt to have summoner traits increase the spawn rating if that is possible. It might be already, never tried.
