You make a good point. McQueary kept his observations in-house until he testified in front of the grand jury. He might not have had time (or access) to meet with the other higher ranking witnesses, to conspire to tell the same version of what really happened.
He probably figured his only option was to tell the truth, and at least avoid a perjury charge. The grand jury thought he was credible.
It sounds like the grand jury believed Curley and Schultz to be less than credible.
I guess it was sort of like ...
Son, we just talked to Joe about this, uh, incident, that occurred late one night in 2002. Something related to Sandusky. Son, can you enlighten us on this? You don’t mind if we take notes, now do ya?