adding Guardian mythic tier to a paladin 6 adds 23 hp

[Penthau]

When I add 1 mythic guardian tier to a paladin of 6th level, she gains 23 hit points, instead of the expected 5 bonus hp. I can toggle the tier on and off and see the hit points jump. When I increase the tier to 2, it only goes up by 5. She has a con of 14, which may be factoring in somehow.