One proven property of tea (especially green tea prepared at a slightly lower temperature) is that it increases arterial flexibility. Contrast that with coffee, which increases blood pressure. I wouldn't be surprised if the effects on the cardiovascular system plays a large role in the calming effect of teas, and the anxiety-increasing effect of coffee.

Which leads to my other point: these experiments always focus so much on the brain without taking into account that the state of the body has a profound impact on the brain too.