Her invulnerability is limited for some reason. Blunt trauma, extremes of temperature, undersea pressure, hard vacuum-- these don't seem to phase her. But bullets, knives and swords have all been shown to hurt her.
In D&D terms, she has DR 100/piercing.
And all Amazons have a measure of super-strength and durability, but Diana and Donna are much stronger than your average Amazon due to the gifts of the gods. Cassie is too, due to her status as a demigod.
In the same way, normal Atlanteans would be much stronger than the average human, but none of them could lift a city block over their head the way Aquaman can.