The scarlet ring lets you swap max health with max focus. Without the ring, I cannot go beyond 900 focus. So I would expect that when I wear it, my max health would be 900. However with a 23% max focus head enchant, max band of focus, and the scarlet ring, my health is going beyond 900.
Without the ring
With the ring
I would expect that the scarlet ring does not allow you have health values that would exceed the normal focus cap. It seems to be using your max focus value before being capped.