Sometimes the stats just don't work right. I have gotten very weird results for books added. Not that I care
but just noticed in passing.
Probably the easiest way is to just add books to the one with the lower number and then delete them till the numbers balance.
You might want to look at the Kobo utilities plugin if you haven't already, as it has some interesting features. One thing I particularly like is the ability to set certain books to todays date which puts them at the top of the recently read list when they get displaced as well as the sorting of series collections.
Helen