How to cope with being a twin?

I'm 13 and a fraternal twin. We are both girls and have pretty much the same friends. We look similar but not identical. I'm ok with being a twin and I love my sister. I just don't understand why people think we are the same person and that we have to do everything together. Why do you think this is? Is there anything I can do about it and will it get better when we are older?