Converting to Kepub with Calibre doubles the page count.

Hi.

I've got a Kobo Clara HD and I use a Calibre to manage all my Epubs. I convert them in Calibre to Kepub and then transfer them over.

One issue I'm having with some epubs is that the page count on the Clara is wrong. With some books it's doubled for some reason. So, something that is 500 pages shows up incorrectly as 1000 pages.

Any idea what is causing this?