I recall something Bobby Jones (?) said, many, many years ago.... something to the effect of, "there is golf, and then there is tournament golf, and neither are one in the same."
I've played with +5 handicaps who couldn't qualify for the local Nike Tour event when it came to town. Yet they could easily shoot mid-high 60's when I played with them on a casual weekend round just a week or two prior at our home course.
Like Mr. Jones said - there is golf, and then there is tournament golf. And neither are one in the same. Shooting 68 in a weekend round with your buddies is a bit different than shooting a 68 in a qualifying round with players as-good or better than you, when you need it to make the tournament.
As my buddy Prof once told me, "Lefty - there are a lot of different levels of "good" in this game."