It's just a transition frame. Her gems were too far apart from each other for her to come together in a single frame.
Yeah... but it seems very strange to me, you never know, they've done this kind of thing before, when the centipeetle was released from it's gem it looked human for a couple of frames, then it was confirmed in Ocean Gem that all the monsters were just corrupted gems, it might mean that a secret about Garnet could be revealed in the next episode, 'Garnet's Universe', or maybe later
Although I'm still unsure about her being a fusion gem... I mean, she would be very weak for a fusion gem if that were the case, being that she only has two arms and is barely taller than pearl.