BUT the Vin leakage is wrong because, I the voltage that I get from Vin IS THE SAME when i measure the 1st and 3rd pin from voltage regulator. Because the results above is the same voltage readings with USB (5.03 V) plugged and the DC jack (3.75 V) plugged.
AND seems the voltage regulator is operating to the Vin pin when USB is plugged because the values measured from Vin and voltage regulator are the same.
If you have no external voltage applied to the external DC connector or the Vin pin (just using USB power) then any voltage you read on Vin is just bogus leakage value as the voltage regulator is not operating (not functioning) at all, just having it's output pin hanging on the boards Vcc bus that is being powered by the USB port.
If you have both an external voltage applied to the DC connector or Vin pin and also have the USB plugged into your PC then the on-board voltage regulator is operating and it then determines the value of the boards Vcc voltage and the USB +5vdc is isolated and not powering the board.
Lefty