If I were to take a guess, those who would wish to exaggerate, lie or merely measure down to the nearest millimeter are only trying to get around general misconceptions about size in general.
Porn usually expresses over-exaggeration toward size. (how many times have we seen a guy who's an honest 7 or 8 being described has having a "foot long" penis, or astronomical numbers being applied to guys with 9 or above? This is, of course, NOT counting the obvious fakes.) I think that causes a great misconception of size in a lot of people. I remember a girl I was speaking with telling em about all the guys she had been with, and the largest being 8 inches. I came to find out that she was greatly overestimating the size of her past encounters.
Now while that overestimation can be a good thing to guys who are truly large, guys who are above average to average will feel inadequate in that situation, not knowing all along that they probably compare or even surpass the expectations of their partner.
Of course, in order to fell better, they may round up the number... they may find out that they measure larger from a certain angle, as long as they get a number that "sounds" good, they can be happy.
Personally, I'm kinda past putting on numbers. It provides too many misconceptions, and really the penis varies so greatly in size and shape that one "9x6" can look completely different than another. This is mostly why I don't put any measurements in my gallery, and just take a bunch of pics from different angles. This way, those who see it can be the judge, and not be confined to a pre-conceived set of numbers.