I understand as a mother that when you hear about a baby who seems more advanced in one area or another, it's natural to worry or at least wonder whether your own child is "okay." But in the end, every baby grows and develops at his or her own pace.
Little bird doesn't roll over but she sits like a pro, while her playmate army crawls all over the place but doesn't sit. Eventually, they both will crawl and sit and it won't matter who did what first. Instead our concerns will shift to "did we baby-proof that room yet?".
If you have concerns about your baby (as most parents do), it is better to learn about the normal timeline for development than to use other babies as a reference mark. Just keep in mind, that even milestone charts are a "general" guide. I always have to remind myself that little bird may be temporarily focusing on one skill rather than another right now or she just needs a little more time. If I still feel uneasy about something, I make sure to ask my pediatrician since they are the REAL experts on baby milestones.