Continuity mistake: In the scene at the beginning of the movie where the flying vampires are chasing the security guard across the boardwalk parking lot, the positions of his arms as he is running changes back and forth between shots. In the close-ups of the guard's face, his hands are up, holding his hat down on his head. In the wide shots, his arms are down at his side. (00:02:45)
Continuity mistake: During the opening we see Michael, Sam and their Mom enter the town and check it out. Michael asks a man if there are any jobs in town. The next time we see Michael he's taking his motorbike off the trailer. Then we see him in the car for one shot. When they reach Grampa's house he is riding the bike. (00:04:55 - 00:06:05)
Continuity mistake: Near the middle of the film we see that Michael has a transparent reflection in a mirror because he is a half vampire. Yet, earlier in the film when the other vampires, who are full vampires, are in the video store, their reflections can be clearly seen in a glass counter. (00:12:10)
Continuity mistake: When Michael first comes home after becoming a vampire he gets some milk out of the refrigerator and when he collapses the whole top of the carton busts open but when his mom comes home and picks it up only one side of the carton is open. (00:37:45 - 00:45:40)
Continuity mistake: When Sam is taking a bath, at one point the bottle of shampoo is there and then missing when Michael enters the room. Then when Sam pops up from under the water, after Nanook attacks Michael, he grabs a blue towel to wipe his face. There was no blue towel previously in the scene, only a yellow one. (00:38:35 - 00:58:45)
Continuity mistake: The family dog bites Michael on the left hand in order to "protect" Sam, which leaves a nasty, bloody gash. As Michael runs up the stairs to talk to Sam, he leans the palm of his left hand on the railing a couple of times for balance. There are no bloody hand marks when he picks his hand up. (00:40:00 - 00:40:45)
Continuity mistake: About 45 minutes into the movie, when Michael is floating on the ceiling then out the window, he knocks the phone off the hook. The phone then rings and Sam answers. (00:42:55)
Continuity mistake: In the cave scene David takes a bite of the noodles which are dry, but when he hands them to Michael they are wet noodles. You can even hear the crunch when David first takes a bite of the noodles. (01:00:00)
Continuity mistake: When the mother is leaving a bottle of wine at Max's house and is being chased by the dog, the dog lunges through the fence. The fence slightly breaks apart by the dog's mouth. The shot changes, and the fence is intact. Another shot and the fences has an even bigger gash in the side, and again, intact.
Continuity mistake: When Max comes over for dinner, and the boys are testing him to see if he's a vampire, Sam spills water on him. As the scene progresses between the liquid spill and Max kissing Mom goodbye, at times they change in size/location.
Continuity mistake: At the very beginning of the film, the family is driving towards "Santa Carla" (for which the town of Santa Cruz stood in) from the south along Highway 1. Then a few cuts later and all of a sudden they're driving in the opposite direction, this time from the north along Highway 1. Also, neither of those routes would be the one someone would take from Arizona.
Answer: Because the Head vampire, Max, had fallen in love with Michael's mom. At the end of the movie he said, "Boys need a mother and if I could get Michael and Sam into the family, you would have no choice but to join us."