Remember too that when you use a tape measure which is, say, 1/3 to 1/2” wide it will distort if wrapped flush around a tapering object. For example, if your dick gets thicker from top to bottom and you wrap a tape measure around it so the edges are parallel when they meet (meaning the tape measure hasn’t been prevented from making an accurate measurement due to being flush to a slightly conical rather than perfectly cylindrical shape) the measurement will be for the thickest part.
Other variations could be due to skin compression (from swelling or just from the measuring device digging in), tension on the measuring device (string stretches), etc.