Two of my campaigns are now at epic levels (the PCs are at level 19 to 20). After some encounters, I wondered about the low CRs of some epic monsters.
Two examples:
1) One party (a half-elf ftr4/clr12/dis3, human pal15/clr4, elf wiz15/mao4, irda wiz14/acm5, centaur drd18/dis1), well equipped...