Hey there. Ashlea was sitting up at 5 months, and my little newphew was at 5 1/2 months. They were both fairy early, Ashlea was just always (and still is) full on and crazy and HAD to be upright and moving around. Ashlea and Jaiden also both had reflux which I think was a factor as lying down upset them.
Another friend of mine's baby didnt start sitting without being held until 10 months.
Its very true, all babies are different