Content Checker:
? When activated, this will check the inventory contents of containers, creatures and NPCs, and the entries of leveled lists to make sure that their entries are of valid types. E.g. it will try to ensure that leveled spell lists only contain spells and other leveled lists.
? If items/entries of incorrect type are found, then they are removed from the container/list. The patch report will list any containers/lists modified and what was removed from them.
Better Imperial City.esp is part of BBC, right?
If you need anymore info, let me know. Maybe I should ask this in the WBash forum?
Yes, ask in the WBash forum, then when you know what it means you can come back and explain it more fully so that we'll understand what it's all about and how it relates to us

There are no levelled lists in "Better Imperial City.esp" and there is nothing in BC with an Editor ID beginning "cob", sounds more like it's from Cobl to me. Also, assuming those are Form IDs (minus the first two digits) that you're listing, none of those Form IDs exist in "Better Imperial City.esp" either, so it sounds very much like this Contents Checker thing isn't working right.