2 different kids.
Even if they were twins you can have this. Even MORE if the children aren't siblings and have different parents.
Milestones and Personality and Inclinations
Is it Nature? Is it nurture?
The answer is usually "Yes. Both."
And it doesn't have anything to do with how smart either kid is. The wisemouth kid may just be average, and the quiter kid may be profoundly gifted. Now, one kid MAY be smarter than the other... but it's really hard to tell with cognitive abilities because there is SUCH a wide range.
If I described my son in some ways... you'd be asking things like 'do you think he'll ever be able to leave home?'... and if I described him in another way you'd be asking if he's going to be one of those kids graduating college before age 18. Like most kids, he excels in some areas and struggles with others.
There are ALL different kinds of intelligence, and there are all different kinds of performance, and there are all different kinds of expectations.
Just as an example... in my family of origin, we got backhanded if we did backseat driving, or immediately taken home (I will TURN this car around was not an idle threat in my family... it was a GIFT... usually my mum just turned the car around and headed home). In my family, my son can comment all he likes on what's going on in the road. I like the situational awareness, and we play games with it. If we'd both been kids together, riding in someone else's car... guess what? I'd have been sitting silent, or answering after pauses, while he'd be chirping away.
Too many variables to answer your question.