Is my 7 month old daughter behind in development?

She is almost 7 months old, i think she is a very smart baby, she can scoot around places to get where she is going she can roll from back to belly and belly to back, but has made no attempt to sit up or get on her knees to crawl, is this normal?? i see alot of people saying there 5 months old crawls.. is something wrong with my baby or is she just being a lazy lil girl :)