It's not particularly clear from the rules, but the explanation we have been given in other threads is that ARM works against all damage that originates from a damage tree result. It doesn't actually have to be from an actual normal attack - it just has to be framed as though it came from a damage tree (by the logic that all damage framed that way is considered to be attack damage).
So "Target models suffers
" counts as attack damage and would be reduced by armour. But "Target model suffers three damage" would not count as attack damage and would not be reduced by armour.
That's how I interpret the explanation given in threads like viewtopic.php?f=4&t=34004
Direct quote from RRG: "The direct damage abilities [that ignore armour] should be written out as text ("suffer 2 damage"), rather than having a