Why is my cat scared of everyone but me?

I adopted a stray kitty (Luna) with my across the street friends Maddie and Piper. She ran up to us when she was a sick and skinny 3 month old. We fed her and took her to the humane society, got her fixed, got her shots, etc. and then adopted her. She's such a sweet kitty, and will snuggle with me and purr, and won't run away from me. She's an outdoor cat, and has fun out there and loves to run around outside, climb trees, and play. She loves me and I love her, and she loves Maddie and Piper, too. But everyone else, she is TERRIFIED of! She will hiss at anyone else, even if they are very nice. Or she gets really scared and runs away. Or she'll start to cry and run away looking for me, or Maddie, or Piper. She follows us, too, sometimes. I was wondering why she does this, being scared of everyone but us. Is it because she was mistreated when she was abandoned as a kitten, and so she didn't trust humans? And then we found her and saved her so she likes us, but only us? Also, Luna won't eat her food if Maddie, Piper, or I didn't get it for her.