Continuity mistake: When Supergirl is kicking the truck guys' asses, the big one has long hair, but in the angle from behind, right before she kicks him, his hair is short. (00:24:35)
Continuity mistake: When Jimmy brings the onion rings, the guy on the left swaps between having his arms down to crossed, between shots. (00:54:32)
Continuity mistake: At the beginning of the movie, Selena drives her car to the right side of the house. When the shot changes, the car is several feet further back and is now heading across the front of the entrance.
Continuity mistake: When Supergirl crushes the rock, her hair is curly and messy. In the following shot it's brushed and straight.
Continuity mistake: When Selena holds on to the fridge door an American flag appears and disappears between shots.
Continuity mistake: After the red haired girl and her friend let go off the monkey wrench, they start jumping and end up close to a steel tank. In the following angle they've moved six feet back.
Continuity mistake: When Supergirl wakes up and sees her bracelet on the floor, in the first shot it is further away from the join between two floor tiles than it is in the second shot.
Continuity mistake: Supergirl lands the colliding car in a shadowy place but a couple seconds later the shadow is completely gone.
Continuity mistake: The mirror in the castle changes to transparent glass when Supergirl flies through it. After the vortex swallows Selena and Bianca, it turns back into a mirror.
Continuity mistake: After the window gets fixed, the position of the cushions around the window differs between shots.
Continuity mistake: Selena parks her car close to the entrance, but in the next shot the car has moved 12 feet to the right.
Continuity mistake: When the vortex breaks the window, the sound of glass breaking is heard but no glass pieces are seen. In a later shot, the floor is filled with tiny pieces of glass.
Continuity mistake: When the redhead girl and her friend fall on the girl's bathroom, watch the girl on the far left. First she is holding a multicoloured towel with circles drawn on; a split second later the circles are there but the towel is white.
Continuity mistake: At Selena's party, while Nigel is talking to the blonde girl, the green straw in his cocktail moves around between shots.
Continuity mistake: At the end, when Ethan is saying goodbye, in the wide shot there's barely no one behind the red car on the left. In the close shot a lot of people appear.
Continuity mistake: Supergirl comes out of the lake but her hair is dry.
Continuity mistake: When Supergirl and the principal enter the girls' dorm, the jogger girl turns around to run away twice.
Continuity mistake: In the castle, when Supergirl asks Selena for the Omegaheadron, Selena turns to her left and opens her eyes widely. In the following shot she repeats the same movement and expression.
Continuity mistake: When Selena makes a toast with Ethan, from the front the logo and name of the beer is visible, in the following shot it's not.
Continuity mistake: When the truck crushes the red car, the woman with the trolley makes a right to protect herself. When the angle changes she repeats the previous movement.
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.