I'm so confused!

Why can't they make things simple when you're pregnant. I don't understand adding two weeks to your pregnancy at the beginning. I am 15w1d according too my chart. So the two weeks we were trying to conceive, are added on. Why? I didn't have a baby inside of me. It's just confusing. And now I have notes saying that this is my 16th week of pregnancy. I don't understand how it can be my 16th week if I just started going through my 15th! Ugh. Confused.