Could someone explain why the 5.825 to 5.875 band of the ISM is not included in the UNII?
The CWNA Study Guide makes a point about the bandwidth differences but does not explain why.
Basically it's because other people/groups etc have it allocated as primary for their use.
Frequency bands are often shared between groups, some with primary, others with secondary, and still others with tertiary designation. There are often historical reasons for many of the bands - often based on who got there first.
In particular, this is one of the reserved ranges for radio astronomy and spectral line observation.
Here is a visual depiction of the allocations:
Here is a table showing more detail with some explanation:
And here is some historical data:
Hope this helps.