From the latest PTS natch potes: Updated the calculation with your Maximum Health, Magicka, and Stamina interacting with modifiers to better reflect the 20% increase to these stats from your first 100 points spent into each tree of the Champion Point system. Previously, this 20% only applied to your base stats, attributes, enchantments, and 2 through 4 piece bonuses on item sets. After this change, all sources of flat stat increases will be affected. Consumable food and drinks, 5 piece bonuses from item sets, and passives will now be properly affected by the 20% increase.
The bold part: it's the first time I hear about this, a flat 20% bonus once you reach 100 CP. Is this new in update 21? I tried checking out UESP and did not find any reference to this.