I have tried replicate it but was unsuccessful. It might have been fixed. It was quite sporadic before though so it might not...
Tested it. Still doesn't work. I have updated the affected versions.
We are having the same problem on our server, though it has nothing to do with the sieges. Every night our server peaks real bad which cause block lag. Whenever we kill off the zombies or they die at daybreak the server cools down and the block lag disappear.
Edit: We're on 1.6.1
I didn't have the same book & sword this time but I got similar results anyway.
Only enchant cost 12 levels.
Enchant & rename cost 19 levels.
Only rename cost 24 levels.
It would be logical that the middle one is more expensive, but it isn't.
EDIT: The sword is a Smite IV Knockback II sword, and the book is Smite IV.
The second picture shows the rename cost without the book. The first picture is with the book. Without the book it's 37 levels and with it it's 23, so it's 14 levels cheaper to enchant with a book AND rename than it is just renaming.
I tried it again on 1.6.2 and it still there.