The USB port does not provide 5V precisely. The specification says it can be as low as 4.4V.
But I measured the voltage of the 1st pin of the mini-USB on the Arduino, and it reads 5.0 volts exactly. Why doesn't the 5.0v output pin also supply this voltage level?
My problem is, I have a sensor, which needs exactly 5.0 volts to function! and I really need the 5.0v pin (maybe I can solder a wire from the 5.0v USB pin to my sensor, but I don't like the idea of wires sticking out of my project ::))