Raven Progressive Matrix: why's my chosen square wrong? Source: Intelligence and How to Get It: Why Schools and Cultures Count (2009). pp. 7-8.


  The example that the problem-solver must follow is set up by the two figures in the top row of the left panel. The figure at the left in the bottom row then specifies what has to be transformed in order to solve the problem. The six figures on the right correspond to the answer alternatives. The solution of the problem requires you to notice that the figure in the upper left of the left panel is a diamond and the figure in the upper right is a square. This tells you that the answer has to be a square. Then you must notice that the bottom half of the upper diamond is divided into two, with the left portion in black. The fact that the left half of the figure on the right is also black tells you that the corresponding portion of the square on the bottom right must match the corresponding portion in the lower left diamond-that is, the entire bottom half must be black. Then you notice that to make the top right figure, one of the bars has been removed from the top left diamond while symmetry of the bars has been preserved. This establishes that you must remove one of the bars of the square at the bottom while preserving symmetry. Now you know that the correct answer must be the square at the bottom right of the answer panel.

My attempt

Between #1 and 2, 2 orange lines remain, the blue is removed, and nothing is added. So in #3, the 2 orange lines remain. #3 has no blue middle line that can be removed. So how can the right answer be #5, where the green line has been added to #3?
As the text you provide explains, there is a flow of thinking involved:


diamond-diamond / square - ? (answer: square - you got that one right)
bottom half becomes filled (you got that one too)
one stripe removed above 


All this added yields the picture at the bottom right. You didn't notice, interpret or actioned on the stripe removal, perhaps.