Actually, that is the total damage given. Without helmet, a headshot can yield a lot of damage, as seen here. Because armor reduces damage by a percentage based on the weapons armor penetrating ability, weapons that 1-shot you, like the AWP, HAS to produce more damage than your maximum health pool, that be 100 health and 100 armor. Also, damage is added from killer, so if one person shoots you once in the arm for 20 damage, then 200 damage in the head (kill), you'd get 220 damage as the number shown here.
In other words, they were shot, probably not carrying armor, thus causing this number to occur. If they had full armor, the number could've been lower.
Could also be the grand total of damage you gave to this person before you died. Most likely he respawned and got killed multiple times.