My DS never crawled. He was meeting all of his other milestones, but went straight from butt scooting to walking at 12 months. I read much of what you're seeing now and can tell you that none of that applies to him. He is now a healthy 7-year-old who is coordinated and has no learning challenges. I think he just figured out that he was more efficient if he scooted, so he did what came easier. That kid was awfully quick on his butt, so I can see why he chose to skip crawling!