A bunch of reasons, but I don't think anyone is lying, just exaggerating and not measuring the way that the pros do.

1) Tour players like Dustin Johnson are measured by their average drive. We all brag about the one shot we hit that far. The longest drive record is 515 yds. Tour players can hit farther than their average, probably a lot farther if they felt like it.

2) Carry distance vs Roll distance are never the same so downhill shots or firm conditions are definitely going to help.

3) Guesstimates are not fully accurate, and neither are free gps apps.

Personally I used to swing as hard as I could almost all of the time, to the point of frustration of several of my friends. Because of this, my distances were all pretty far, but my accuracy was dismal. I have started working on controlling my swing, and now have subtracted a fair amount of yardage from every club to adjust.

Example: Me and my brother (27 and 20) were playing at a well maintained country club this past week from the whites. On hole 18 par 5, 567 yards (http://www.pointjudithcountryclub.net/club/scripts/golf/View_Course_Scorecard.asp) we each took a second drive to see how far we could hit if we swung all out. I hit 339 and he hit 341 according to my free gps app. I put my second shot with a hybrid just shy of the green, short chipped, then 2 putted for par. My brother double bogey'd. Does this happen all of the time, absolutely not, that is the farthest either of us have ever hit, ever. I shot in the high 90s, he shot in the low 100s. Point of the story, flukes don't mean people are consistently doing this all of the time nor are they lying and we would likely all benefit from relaxing and focusing on mechanics instead of killing our drives.