Please explain......

On a normal week day, we put Carter to bed anywhere from 9:00-10:00. He sleeps all through the night, and I have to wake him up at 7:00 in the mornings to get ready for the day. On the weekends, if we put him to bed at that time, he will usually sleep until 8:30, on a good day. Every single time I try to keep him awake a little longer and feed him later, with the hopes of him sleeping a little bit later on those Saturday and Sunday mornings, he's up EARLIER. Like this morning, 6:50. Before the time I would even be waking him up on a week day. Can someone explain this to me?