It's the only way the numbers work. If you've seen other stuff, please, point me there.
Keep in mind, I could be wrong. I would guess that if Paragon grants base 20, then Epic grants base 15, Heroic (of course) base 10 for Defenses.
From the Pit Fiend stats released a while back. Sorry if someone else already cracked these:
Pit Fiend ability scores have 1/2 level figured into total bonus, so subtract key ability from DC
number like this:
AC 44 keys from STR bonus +24 so base DC is 20; Paragon base is 20
FORT 42 keys from...