Continuity mistake: Supergirl crushes a rock and gets dirt all over the sleeve of her suit. A second later dirt is gone and the suit is clean.
Continuity mistake: Selena starts to say that everything is history and turns around to say the world is hers. Right then, angle changes and cupboards appear on the right from nowhere. When the angle changes, the cupboards disappear.
Continuity mistake: Selena says "Bad luck" and places her hand on the fridge door. In the next shot, only the tips of her fingers are on it.
Revealing mistake: When Supergirl falls on the planet ground and gets released from the mirror trap, she gets up and the hard floor stretches down, revealing it's a rubber mat to soften her fall.
Revealing mistake: When Supergirl is writing her letter of recommendation at super speed, it's way too obvious that the scene's been speeded up in several takes because the leaves in the background move fast and in a very weird and unnatural way.
Continuity mistake: While Selena is burning the chicken, in the close up shot big flames come out, but a split second later, from a different angle, there's just smoke.
Continuity mistake: When Supergirl enters her dorm, the Cornflakes box changes position from standing upright to sideways.
Continuity mistake: At Selena's picnic, the label of the champagne keeps turning to face the camera.
Continuity mistake: When at home, Selena says that the world is at her wish and looks down at the bronze sculpture which she is holding close to her chin. In the following angle her hand swaps to stomach level.
Continuity mistake: Supergirl's bracelet swaps wrists while she's flying at night.
Continuity mistake: When Selena is holding the glass champagne and says "I can't wait 'till the world is mine", from a back shot her finger is below liquid level, in the following shot it is above it.
Revealing mistake: When Selena uses her magical powers to accelerate the fairground ride to superfast speed, the twinkling fairground lights in the background are also flashing much faster. They obviously played the tape back at high speed to achieve this.
Answer: It's never explained, she could have changed at super speed. In Superman II, when confronted by the super criminals at the Fortress of Solitude, Superman did a disappearing and reappearing act. So did the criminals. It might be a similar thing.