Why is every baby I see...

You shouldn't compare children. Each child developws at their own rate. That doesn't mean that you are doing a bad job parenting. If you are concerned I would have a talk with your daughters doctor to express your concerns.

I think it's natural instinct for parents to compare their children, but remember that there is a wide range in terms of growth, both physically and intellectually, especially with babies. If you are worried about her growth progress, talk to your pediatrician to make sure she is being charted properly. As for her intellectual growth, every child develops at their own pace regardless of stimulation. They take from their environment what they need and if you are providing her with appropriate stimulation, then she will develop at her own pace. This has nothing to do at all with how smart she is. Don't underestimate your child. She may very well be quite smart in the long run and the other kids may be trying to catch up to her at some point.