Why won't my instructor/BOSS let me ride ?

I went to horse camp as a kid. I've been riding for 7 years (Since I was 6, now I'm 13) And I started going to this Lady's barn a bit over 3 years ago. I only went to camp on the breaks from school, and occasionally on a Sunday. This past year and a half (Since about christmas of 09) I was been really into the camp, going as often as I can and trying really hard but I couldn't go too often because of financial issues in my family. This past summer I went to camp almost every day all summer long because we finally got some money. this lady who runs the barn has very few older kids at her camp, so she remembered me. This January she called me and asked if I wanted to work for her because she runs a party-pony business and needs help. So i'm at her house all the time now, and I get to ride for free alot (as well as getting payed). Sometimes she even takes me and another girl out to the park or lake and we ride there. but the thing is, this girl is my same age and has only been at he camp for about almost 2 years and just started riding when she joined. She;s kind of really overweight (sorry I had to say it) so you can't really see her posture when she rides or anything, but for some reason she's treated like an adult and does whatever she wants. She leases the horses, rides them , does whatever whenever. Well today we were at the lake, Me and her and my brother who doesn't do much and we were walking around in the water and decided to ask if we could take off the saddles. well our instructor/boss says right to my face, "Oh ___ you can go ahead and take Beau but sorry Nicole (me) you can't." and she NEVER lets me ride . She has 2 green-broke horses (ones only 3 and still learning and the other was an abused jumper before we bought him so hes super spooky) and only very few people can ride him and she lets me ride either of them whenever I want, but not even the smallest most beginner horse she owns . Is there a reason to this? I must be a good rider if I ride the 2 green horses and don't get thrown off all the time, right?