I currently have an Intel NUC7I7BNH and I’m thinking about upgrading to an Intel NUC8I7BEH.
The thing is that the NUC 7 uses 2133MHz memory and, according to Intel specs, the NUC 8 only accepts 2400MHz memory, except when you browse through the compatibility list and see a couple of 2133MHz memories.
What’s the deal with that?
You're reading too much into it. There's a maximum validated (and warranted) speed and everything less is implied if not stated as being supported.