Question about my Pit Bull/Lab mix?

I have a 5 month (on the 14th) old Pit Bull/Lab mix puppy. She's a good dog, although she likes to challenge me a lot, which I am working with her on and she's starting to get the point that she's not the alpha here. I'm curious though. When she's out and about in the house, she likes to get attention from everyone, but when I leave the room, she'll follow me. She goes in my room on my bed with me while I play on the computer she sleeps or chews on her toys. She follows me in the bathroom etc. etc. Why does she follow me so much? She's done it since she was very little. She also only listens to me. She could be going after something and my step father could tell her to "leave it", but it's like she ignores him. But as soon as I say "Mya, leave it!" in a stern voice, she walks away from it. I do most of her training, I feed her, I basically do everything. Which I know is suppose to give you the alpha status with them. But, we also have a dachshund that I do the same for, but she doesn't listen to me, is scared of me (everyone for that matter) except my mom, she follows her everywhere. So, what's the difference?

Also, Mya is 39 pounds right now at almost 5 months and she's lean, not over weight. I just got her spayed a few days ago, does anyone know how to determine her adult weight? I found a place where you divide the weight she is right now (39lbs) and her age in weeks (21) and times is by 52. The answer was 96lbs. That is HUGE. The vet had said no more that 50lbs - 60lbs...any guesses? Thanks!