In my experience different people respond in vastly different ways to the same pot. The same is true of alcohol of course, but I believe variation in response is greater with marijuana than it is with alcohol, especially when it comes to motor skills. Can I prove it? No. But if I were a researcher I'd start asking questions toward that end.
Not to mention that pot itself can be highly variable in its chemical composition (this is a feature, not a bug), so any kind of blanket statement about the effects of marijuana on a human being is bound to be rather approximate.
I've known people who insisted that they functioned (whether behind the wheel or not) just as well high as they do straight. And I believe them, having spent quite a bit of time around such people, including being sober around them. IMO there are a few such people out there who wrongly assume that their own response to marijuana is the norm, and are quite vocal about it.
Well, I know that's not how I respond. I and many other people can barely open a bag of potato chips without becoming a danger to ourselves and others. Getting behind the wheel is the last thing I want to even consider in that state.