The first THOR movie featured an Infinity Gauntlet in Odin's vault, while Avengers: Age of Ultron showed Thanos with that weapon now in his possession. However, by the time Avengers: Infinity War rolled around, we learned that the Mad Titan had forced Eitri to create the Gauntlet for him, creating something of a plot hole in the Marvel Cinematic Universe.
Thor: Ragnarok would later reveal that Odin's Gauntlet was a fake, but that still doesn't explain what the deal was in Joss Whedon's 2015 movie.
Well, writers Christopher Markus and Stephen McFeely have now shed some light on that by revealing that the Infinity Gauntlet Thanos donned in that post-credits scene was just a "practice gauntlet." However, when he needed the real deal, that's why he got Eitri to make one for him.
This is still an explanation it would be easy enough to pick holes in, but it's as good as it's going to get at this point, and it does make sense! Plus, there being more than one Infinity Gauntlet in the MCU would have made the mission of Earth's Mightiest Heroes far too easy in Avengers: Endgame!