On my third playthrough I have a group level 12 with 19 xp out of 20 needed for the next level. And seeing that I'm scratching my head, because I don't remember having seen an odd number in xp in any of my previous playthroughs. I though fighting a map at your level gives 10 xp, one level below 4 xp, and two levels below 2 xp. Does winning a battle above your level give an uneven number of xp?
I think completing an adventure one level above yours gives 15 XP. I'll see if I can dig out Sir Knight's handy XP table... Edit - Here it is: I think these are all still correct.