Kepub question

When I view my single converted kepub on my aura one, it is missing / cut off a few pixels at the right hand margin. Is there an obvious reason?! The wpub displays fine. I tried a few things like changing margins and reconverting but it doesnt help.

Also is there a kepub spec anywhere? I was just browsing round and the only info I could find was this
https://wiki.mobileread.com/wiki/Kepub
I had a quick look at my converted kepub and it does have spans round every sentence.

Ps so what's a cruller doughnut then!