I understand that this doesn't take a significant chunk off of the entropy involved, and that even if a whole nother character of the GUID was reserved (for any purpose), we still would have more than enough for every insect to have one, so I'm not worried, just curious.
As this great answer shows, the Version 4 algorithm for generating GUIDs has the following format:
xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx
The RFC spec for UUIDs says that these bits must be set this way, but I don't see any reason given.
Why is the third bullet (the 17th digit) limited to only those four digits?
Quoting the estimable Mr. Lippert
First off, what bits are we talking about when we say “the bits”? We already know that in a “random” GUID the first hex digit of the third section is always 4....there is additional version information stored in the GUID in the bits in the fourth section as well; you’ll note that a GUID almost always has 8, 9, a or b as the first hex digit of the fourth section. So in total we have six bits reserved for version information, leaving 122 bits that can be chosen at random.
(from https://ericlippert.com/2012/05/07/guid-guide-part-three/)
tl;dr - it's more version information. To get more specific than that I suspect you're going to have to track down the author of the spec.