I really have thought about that part too...and I think that this is another thing that does not fit! In my opinion, the memory of his father shouting at his mother while he is crying in the corner is much more personal than the one he actually tried to hide. Why didn't he hide them as well? Itwould only explain the whole thing to me if there was more to the memory in the pensieve than we know by now. But then, why did he hide it that obviously, in front of Harry's eyes?? He could have just locked them in the pensieve a minute or so before Harry arrived. Or at least, he could have locked the pensieve away. But no, he almost invited Harry to walk around in his most private and harmful memories....
That doesn't make sense, and I refuse to believe that JKR wrote that just for the sake of getting the story going.