It actually isn't device why I ask
It is actually for accessibility reasons. The ePub 3 doesn't mess with things like tables, etc. as much as ePub 2...it can be a real problem if one uses a screen reader to 'read' the file. I wish I could use ePub 2, because this product is great...but I don't think I can. I need to do more testing to confirm this though, so maybe ePub 2 will be okay in the end.
Thanks!