Why does Ed tell Lewis they had to change their story?

After Ed, Bobby and Lewis make it through the last set of rapids, they agree to tell the authorities that everything had happened in those last rapids. Lewis even struggles to tell Ed that he understands the plan exactly. They do in fact end up telling that story. Why then, when Ed and Bobby go to see Lewis in the hospital, does Ed lean in and whisper to Lewis that they had to change their story, when in fact their story hadn't changed at all? It was the very same story Lewis said he understood and agreed to earlier.

Did Ed not trust that Lewis had the faculties to remember what they had agreed to, and so used that as a signal to Lewis to just plead total ignorance?