Cruise Control problems of the 2013 Ford Taurus

While driving the vehicle at highway speeds, it was noted that when I set the cruise control at 70mph, the stated speed limit, the digital numerical readout read exactly 70mph. The speedometer although reads consistently 3-4mph above the cruise control setting. This has occurred since I took delivery of the vehicle as brand new from the dealer and occurs regardless of speed setting, I. E. : same issue if cruise is set at 45mph, 55mph, 65mph, etc. After becoming concerned that the vehicle may in fact be traveling faster than the digital cruise control setting was stating, I took the vehicle back to dealer. They were unable fix the issue with exact explanation on my repair order reading: "client states when cruise set the speedometer always 3-4 miles higher. Nwp contact Ford. Advised to drive 2013 and 2014 models to verify concern. Both models same symptom. All ok at this time. ".