Hi,
My son started to crawl a day before turning 9 months - however he had been trying for what seemed like AGES!!!!
My nephew turns 9 months in a day or 2 and is rolling everywhere and not crawling yet despite plenty of tummy time. There's also a couple of babies in my mothers group not crawling yet. I am pretty sure that everything is fine as long as they are trying to move by 11 months or so so you have nothing to worry about (although I am not sure you are even worried - maybe just curious?).
Apparenetly I bum-shuffled as a baby and not even that until 13 months and here I am reasonably normal!