Mmmm .... I don't know how things work, but I do know that they're a tad confusing!
Based on my 'old' library screen I haven't bought a book for that account since May 2015, but my 'new' library screen informs me that I bought 32 books on the 24th April, 2016. Clearly tosh!
And I didn't import any of my Sony account books to Kobo, so that can't account for it, and they are a mixture of DRM, DRM free and 'Buy now' .....