The higher the monsters the more they deviate. Orcus is listed as CR 26 but is probably CR 20-21 for instance.
Orcus CR check
Defensive CR:
HP = 405 x 1.25 (immunities) + 90 (legendary resistance) = 596.25 EHP
AC = 20 (with wand) + 2(saving throws) + 2 (magic resistance) = 24
Defensive CR subtotal = 27
Offensive CR
DPR = 158 (2 wands & 3 tails)
Attack bonus = 17 (average of all attack bonuses)
Offensive CR subtotal = 24
DMG Total CR = 25.5 (27+24/2)
Seems pretty close to the CR 26 listed to me. And that doesn't account for it being able to summon 500 HP of undead warriors if needed.
Now a few issues. The DMG provides guidance for applying the resistance and immunities multiplier to HP, but it doesn't say you have too. I think a 1.25 multiplier for this high CR is a bit much, but it has immunity to 4 conditions and 3 damage types (lumping b,p, & s as one), so it feels justified to apply the lowest option provided by the DMG (and WotC seems to agree). Also, I often find that the MM rounds up fractional CRs, when I think they should round down, but as far as I know there is not guidance on this in the DMG. I would list Orucs as a CR 25, but the Wand of Orcus probably justifies the higher CR.