My guess is that it's probably the same reason some companies have been able to put out Beholder and Mind Flayer products...
They asked. If you get permission from WotC to use it, whether it's in the SRD or not, you can use it. You may not be able to declare it OGC, but you can use it.
Remember, the OGL does not trump copyright law - it simply says, "without you needing to ask, here are copyrighted things we give you permission to use, provided you stick to certain guidelines (the OGL)." If you want to use other copyrighted material, you can still ask for permission as normal.
In fact, you could conceivably publish a book full of stuff that had been published elsewhere as OGC without using the OGL - you'd just have to get explicit permission from every person who held the original copyright to begin with.
To sum up...
You can't copy copyrighted material and publish it unless you have permission from the copyright owner.
The OGL is one form of "blanket permission" so you don't have to ask on a case-by-case basis.
Whether or not something is OGC, you can always ask for permission - and doing so does not violate the OGL (unless you declare it as OGC when you're not supposed to).
--The Sigil