My first was late to roll, move, walk etc etc. I used to take her to playgroup and while everyone else's baby was rolling around, pushing up or whatever she would just lay there.
Spoke to my GP about it at one point, as like you was worried she wasn't developing in line with the books and she said much the same as all the other posters, each baby is different and they all develop different skills at different times, there are very few babies who comply with the averages. Plus, she said that most developmental timelines were drawn up years ago, when babies used to sleep on their tummies more. This meant on average they pushed up earlier and moved earlier than back sleepers.
My second was the opposite. Rolled, crawled, walked etc early. No.1 was only in cloth from 4 months, No.2 from birth.