Hi Bonny Gay, you are right that every child is different & will do things when they are ready. My youngest was crawling & walking earlier than her elder sister but there wasn't too much difference between them (youngest started crawling at 5mths, cruising at 9-10mths & walking indepently at 11mths whilst eldest started crawling at 8-9mths, cruising at 9-10mths & walking indepently at 12mths).
There are many reasons why children are the way they are but if it does cause you concern then talk to your HV/GP. They can either reassure you everything is fine or refer you to specialists to determine any underlying causes.