Summoned Skull is not a Warrior, Gaia the Fierce Knight Origin is only treated as 2 tributes for a the tribute summon of a warrior, the issue is when you are tributing a card that can be treated as 2 tributes for a monster that only requires 1 tribute. The problem is even more pervasive than Gaia I have found, if you would try the same Summoned Skull test with Dark Effigy instead of Gaia you will run in to the same problem I had with Gaia, I think.
Here is a similar situation with Earth Effigy that should be treated as 2 tributes. The only things to happen this game were normal summon effigy, an attack, a set of effigy then an attempted tribute summon which could not find materials as it tells me I need at most 1.
