Unfortunately I'm still unable to replicate this, at least based on the info given.
I made a human warpriest of Nethys. For easy work I set all stats to 18 before racial and gave the +2 to Con to make it 20. I set automatic max HP at new levels and left the FCB on +1 HP. For Feats I took Toughness, Ignore Pain, Combat Casting and Endurance. Bonus feats for lvl 6 warpriest are combat reflexes and defensive combat training.
At level 6 with Mythic turned off, I'm at 90 HP.
Turning on Mythic and staying Tier 1, adding just Guardian I go to 95.
I took Absorb Blow for my Guardian's Call, no change to HP (as expected)
For the Path Ability I took Armor Master, still no change to HP (again, as expected).
For Mythic Feat I took Toughness and my HP Went up to 101, which is, again, as expected.
Not sure what may be adding the extra HP to you, but Guardian itself isn't doing it for me.