You make a strong attack.

Effect is more important than numbers. In this case you onlt did 2 damage. However, the spider only had 2 hps left.
As you can see in the picture, you did 100% damage, and the Strike Orb is at full intensity. This provides positive feedback for the player, and also lets you know you successfully took all hitpoints.
The orb could also show (through different colors and effects) critical hits, and other special circumstances.