Why do you think this is?

i have trouble making friends or relating to people mainly because i don't have a lot to say and i'm shy but i don't have problem speaking in front of a crowd.. in school every time i had to get up and present my project and everyone was always surprised they all said they thought it would end up bad but i managed to be calm and articulate and now they think i'm pulling a double persona kinda thing.. but it's not true.. and people always think i'm lying or having a secret life behind their back or something, why is this?