This statement is made by Sheriff Heck Tate to Atticus. Atticus is convinced that Jem killed Mr. Ewell in self-defense, but Mr. Tate insists that it did not happen that way. In the statement in question, Mr. Tate is acknowledging that Atticus has "been under a strain tonight no man should ever have to go through," and he is surprised that the strain hasn't been enough to leave Atticus unable to function, which is what he means when he says
"Why you ain't in the bed from it I don't know..."
Mr. Tate is insisting that Mr. Ewell fell on his own knife and killed himself, but, because of what Scout said, Atticus is convinced that Jem killed Mr. Ewell while trying to save his sister. When Mr. Tate keeps saying that Mr. Ewell caused his own death, Atticus thinks he is making up a falsehood to protect Jem, and he does not want Mr. Tate to do that; he is intent on being completely honest about what has happened. What Atticus does not realize is that Jem is really not the one who killed Mr. Ewell; Boo Radley did. Mr. Tate is indeed making up the story that Mr. Ewell caused his own death, but he is not doing it to protect Jem, he is doing it to protect Boo Radley.
Mr. Tate is frustrated that he cannot make Atticus, who is usually very discerning, see what he is really doing. He attributes Atticus's inability to understand to the strain he has been under, telling him,
"for once you haven't been able to put two and two together."