Why do people mistakenly think I'm a mean girl?

It happens to me ALL the time. A ton of people who are now my good friends have told me that when they first saw me they got the impression that I was a mean, snobby, high maintenance person because I like to dress up and stuff like that but then when they actually talked to me they liked me alot. Why do people judge so quickly just on first sight? I've worked at a place before where the manager who had never even really spoken to me before blamed me for something I had nothing to do with and lashed out saying, "If you spent half the time you do on your hair extensions and colored contacts & spray tans maybe this wouldnt happen!" I don't even have extensions or colored contacts or fake tan and never have in my life lol. Recently my guy friend told me that when he saw me walking through his apartment complex he thought I was a "Worthless, snobby, gold digging ***** b/c the people I was hanging out with he said were "too ugly" so I must have been using them. but when he hung out with he he knew he was very wrong. I'm probably one of the nicest girls you could ever meet but people will make the mistake of thinking I'm really mean without even talking to me first. So why is this?