Page # wackiness (epub on Kobo)

I have a book whose page numbering is unright. It starts out 1 of 4, 2 of 4, 3 of 4, 4 of 4, then for a while alternates between 3 of 4 and 4 of 4, then starts counting up 5 of 4, 6 of 4, etc. When it gets to a new chapter, it resets to 3 of 4 then starts counting up again. The correct pages show up in the margin.

I looked inside the file with Sigil, but I don't know enough about how it's supposed to be structured to see what might be causing this. Any notions?

Thanks!