My son never crawled. Instead, he slid around with his buttock touching the floor. As he sits alot, he is very focused & can finish few books & puzzles per sitting. He finally walked at 18mth
My gal crawled at a few mths old. She can't concentrate at 1 activity as long as her bro. Instead, she developed better fine motor skills earlier. She walked at 12mth
Guess all those milestone guides r just for ref. I'm no pd, so from a mum's pt of view, let ur child grow at his pace. Encourage them when necessary, but dun be over alarmed by casual remarks. But if it is professional advice, mb can seek 2nd opinion