As shown in
this picture from
this page, at some point in the timeline the necro epic had +40hp and +20 mana (currently has +20hp and +80mana on P99).
The change may be related to changes in the stats on the warrior epic as mentioned in
Loraen's post; I remember reading somewhere that all the epics were balanced to each add a total of 100 mana/hp across the board, but I don't know the details.