On PSN, Square Enix sold Heavensward up until the release of Stormblood, but didn't explicitly say that Stormblood included the first expansion tacked on? So, if people bought Heavensward right before Stormblood was released, they would be out about $20-$30, considering that Heavensward comes included with Stormblood.

On another note, what happens when you already own Heavensward? There's no option for a standalone upgrade to Stormblood.