Spiders have been dealing 3 HP, rather than 2. The Minecraft Wiki says they deal 2 HP on normal difficulty.

How to reproduce:

1. Make a new Creative world with normal difficulty
2. Spawn a spider and set the time to night
3. Set the game mode to survival
4. Take off any armor if you have any on
5. Let the spider attack you

Observed results:

The spider deals 3 HP

Expected results:

The spider should only deal 2 HP