The answer to this question can be found in the very beginning of chapter eleven. Jess has just been told that Leslie was found dead by the creek that morning. Jess automatically assumes that his family is trying to tell him that Leslie drowned, and Jess immediately denies that as a possibility. He knows that the likelihood of Leslie drowning is slim because she is such a strong swimmer:
"No," he said, finding his voice. Leslie wouldn't drown. She could swim real good."
Mr. Aarons immediately follows up Jess's comment with further explanation and details in an attempt to make Leslie's death more believable and real to his son. He does this by explaining that the rope over the creek broke. Leslie was likely swinging on the rope when it broke; therefore, the fall happened at an odd angle and increased velocity. Mr. Aarons tells Jess that the people that found her believe that she likely struck her head on something during the fall. Jess and readers are meant to assume that Leslie was knocked unconscious and wasn't even able to swim for her life:
"That old rope you kids been swinging on broke." His father went quietly and relentlessly on. "They think she musta hit her head on something when she fell."