I find it reasonable. It's complete possible to measure a dick with an accuracy of 0.25 of an inch (0.6cm). I can.That's a pretty exact measurement, which equates to 27.4cm - I'd be amazed if you could measure a guy's dick, in inches, to that degree of accuracy.
Think about it, 0.4 of an inch...? C'mon!
My variation in erected lenght differs no more than 0.5cm ( so less than 1/4" ). But ofcourse allways measuring with the same methhod : Standing up, holding the penis as much as possible parallel (horizontal) to the floor, measuring on top, from pubis bone (without pressure) upto the tip.