The bum scooting is considered the crawling stage for this child. Some kids just do it differently. Think about that pampers commercial with the kids moving and there are some scooters in that one. My one daughter was a scooter, others continue the commando stomache crawl longer than infancy and others rabbit hop as in hands first and then the feet leap up and join the hands and while others actually do the alternating hands and feet we expect.

It is hard to determine "normal" now that we don't get them till a year since we are up against different parenting styles. What you are observing is more related to learned behaviours at home and how to get what they want or what is expected of them than anything developmental.