There's two reasons why his statement to Joe that he was ok would be more reliable than his testimony as to how he described the conduct to Joe/Curley/Schultz. First, the statement to Joe that he was okay with how it was handled, is a statement against his interest. It doesn't serve him any benefit and in fact undermines his story somewhat. Generally, those statements are deemed more credible (in fact, such statements are an exception to hearsay since they are considered more reliable at law). Second, I think MM was pretty clear that, when describing the conduct to Joe, Curley and Schultz, he couldn't remember the exact words but recalled the severity what he conveyed. That's a huge distinction and leaves a lot open to interpretation as to what MM thinks he may have conveyed (10 years ago mind you) versus how Joe/Curley/Schultz interpreted what he conveyed.
As I've said, perfect example of people twisting McQueary's words, then blaming him for changing his story. He TESTIFIED under oath that Paterno was asking about his mental health when he asked if he as ok, he was not asking about whether McQueary was ok with how Curley and Schultz handled the incident.