Well, going with the assumption of 8th level:
Rogue: 20 Dex (5), with the half level added (9) +2 Leather (13) with half level added a second time (17) and the base 10 (27) gives the value he mentioned.
Cleric: 18 Int (4), half level (8), +2 Hide (13), half level (17), plus 10 is 27 as well. Or perhaps the Int is a bit lower, and he spent a feat or two on shields [especially if he's a battle cleric]. A high inteligence would fit the wisdom based "cloistered cleric" type, not to mention that Religion is Int based so it helps for ritual casting.
So, while it may not be confirmed, it's likely the "adding half level to Dex/int modifier" theory is how they are increasing their AC "every level" as the OP put it.