Ya, mine too, though I killed the arch demon with the kill animation and everything. DAO has quite a few little bugs like that. They don't interfere with game play but much like the char sheets from Diablo II, they are a source of misinformation or lack of information altogether.
just an update, as the total damage done become larger, ie: 1006079, ( 7 digits now), the total damage contribution drop further, from 99% to 96%.
Right now, my contribution drop to 87%.
it seems after it hit 999999, additional damage will reset the counter and those damage dealt after 1million will be deducted from the 100% contribution.
simply put, the damage counter starts again from 100% and goes backward.
2nd set of 999999 damage will make the contributed damage goes from 100% --> 1%