The BTC price of goods only remains constant if sellers decide so. And they generally have a supply chain and labor that is heavily impacted by exchange rates. Plus, neither consumers or producers are ignorant of exchange rates, so there aren't going to be products marked at 1 BTC forever if BTC appreciates/depreciates.

But there's nothing within BTC itself that's volatile. The currency is slowly being inflated to the target 21 MM coins.