That generally seems to apply to the open world as well.
I've been item hunting on Hell's Ridge for a long time now, always circling the same large plains north of the starting island entry point, over and over and over and over again. And indeed, this too had rounds of nicely high level beasts, 233-234, which then gradually went down to around 227-229 and back up, almost like a sinus wave. (that was with a 216 Daemoness, 97-98% SB)
Granted, my view on this may be awfully biased as I was hunting for the highest possible item levels, but here's my suggestion for this spawn behavior:
To put another layer of uncertainty to the RNG of drops, effectively drawing out the process of finalizing a character in terms of gear From a developer's perspective, that would actually make sense~ if the relation between char level, SB and enemy level was fixed, absolute, you'd be done gearing up so much quicker!
Here's an example: Pandora's set items, or the free weapon sets - they drop at 210, 225, 240 (and theoretically 255)~ Now with your 216 toon, the usual drop would be a 225 one since enemies of, let's say 227-232, are closer to that than the 240 drop. However, as the spawn cycle reaches the top end, 233-234, those hot 240 drops are on the menu! And once you got a few of those, you can't be satisfied with puny 225 ones anymore