Why do cats do that

This one is about buggie. Our Cat. She has a habit of jumping out the front window, bouncing round to the back garden to the back door, jumping through the cat flap , walking through the house then doing it all again. We decicided that it must be fun for some reason so we both decided that Lynn had to have a go to see what all the fuss was about. She squeezed through the front window. It was a bit tight on her bum. Her bum its not big, its the the window that's a bit small. I heard the cat flap go and then Lynn appeared back in the front room. I think she cheated on the cat flap. I am sure she just came through the door. Her bum definatly would not fit through there. We are still not sure why the cat does it.