The pricing of anything is always associated to the law of "supply and demand." Simply stated, if buyers weren't willing to pay the higher price, sellers would be forced to make an adjustment.
Without knowing the area, one could only speculate that there are desirable features that are attracting buyers to pay the higher price in Potomac. This could be related to location, proximity to employment, amenities, social, recreational, & cultural opportunities etc. Buyers are often quite willing to pay a little more for a higher quality of living that one location offers over another.
Hope this is helpful.