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Martin, I ́ve explained it in my other dissue, issue 8411, with a step by step example.
It is unfair because a thread can _bypass_ the condition variable. A thread just woken up from the condition variable has to race to get the lock, and it is a race that it will invariably loose if the other thread is doing a release/acquire (yielding the GIL as happens in ceval.py) The ConditionVariable can only endow the lock with its fairness property if all the threads play by the same rules.
This was a design decision made by Tim (according to the comment) but a misguided one. It is a good stragegy for resources that are held for a short time to avoid lock convoying, but not appropriate in this case.
You also don't have to take my word for it. Just try it out. Notice that 99% of all yields between threads fail, causing starvation of a thread which is the definition of "unfairness."
Anyway, this is the last time I explain why the "emulated" semaphore is unfair. I think I ́ve done so on at least four or five different occasions and it would be helpful if people would actually bother to read my comments.