Each baby's different, suppose its just a case of them strengthening their centre of gravity. We would always sit Oliver up initially, he didn't crawl till 12mth and didn't roll at all until he was crawling, thinking back he was a lazy bum
I would have thought when he figures the rolling onto front, and crawling it all seems to happen at the same time thats when he'll start to sit up himself.
Saying that Oliver sat up on his own in his cot at 7mth (started sitting unaided at 6mth) but he didn't learn to pull himself up to standing until 10mth.
We'd always had the cot on the 2nd (middle setting) it was when he was pulling himself up to standing we moved it to the bottom.