The descriptions here are showing you every place from which a
completion string was gathered, but then all the completions are
lumped together at the end. (That is, you get all the descriptions,
and then all the completions.) If you actually want the completion
strings to appear immediately after each description, you have to add
zstyle ':completion:*' group-name ""
My bad. I used to have this on, but later turned it off in an attempt to minimize the amount of configuration changes and forgot about it.
I think it's time to close this thread. The analysis of Ctrl-X ? logs would take (I guess) a few days of studying the inner workings of the completion system and I cannot afford such a time allocation at the moment. I'd like to thank all who answered and namely Bart for the advice and patience.