It would stand to reason that players would improve dramatically from spring to fall camp. Or maybe from fall camp to Week 1. Or heck, perhaps even during bowl practice. But none of those statements make for an effective cliche, so if you ask a coach, they'll tell you without hesitation that the most progress your team makes all year is between Week 1 and 2.
And it's true. Look around the country. Michigan got better. Penn State got better. North Carolina got... OK, it's not foolproof.
But the point is, if Week 1 is a litmus test of teams' biggest flaws, Week 2 is our first opportunity to see if they've been addressed.