They say that pickups too close can do that as well, but he was talking about the harmonics not fundamentals.One reason is that when a string is plucked/picked, especially when done so with "passion", the string actually does go sharp a bit at first, then settles down to the correct pitch. When tuning a string it is always a good idea to either pluck/pick lightly, or wait a second for the string to settle down before judging the accuracy of the tuning.