Crawling would be a little advanced at 7 months (9 months is the "normal").
With any milestone, the age given is the age at which the "average" infant can to do it. That leaves a pretty wide range. For sitting without support, the milestone is 6 months. In general, as long as the milestone is met before the next milestone is due (in this case, at 9 months) it still falls in the normal range.