I've been 100% in tune with my first kid's milestones.
He was on time for keeping his head straight. A little early when it came to sitting up. Clapped his hands according to expectations. A little behind on drinking from a sippy cup, but a little early for using straws. It took him a while to lift himself up. While the books said he was supposed to crawl forward, he only crawled backwards. He was walking at 13 months--pretty much on average. He was an early speaker, which was particularly advanced because he spoke two languages. He was behind on jumping. He was within range when it came to potty training. Late on using his bike pedals. Early on puzzles.
And now he has a little sister. She'll be a year-old soon, and I still have no idea how she's doing. One day I notice her smile, then she claps her hands, and then one day she starts to sing, and then crawl, and all I can think is, "Oh cool, I guess she can do THAT now."
And don't get me wrong--I think that's a good thing.