Why is my daughter harder to potty train then my boys?

I have read a few post where a lot of people say that girls are easier to potty train than boys. I have three boys and one girl. My two oldest boys were potty trained almost perfectly in a days time, my other boy isn't even close to potty training age yet so he doesn't count yet. My daughter I have been trying to get going with potty training and it's been a week and has more accidents than not. She is very intelligent and has actually been more advanced than the boys in other things like talking. So if potty training is suppose to be easier in girls why is my family so different than everyone elses? I know she is ready to potty train because she knows when she has to go potty.