I just can't find my error, so I hope someone else can give me a hint

I try to transform the image a4p3b into the plane of a4p3a. Therefore, I proceed as stated in the task. First, I use the homography to get the coordinates of a4p3b in the plane of a4p3a. Afterwards, I use interp2 to interpolate the pixels of a4p3b at the calculated coordinates . My result, however, is not as desired. It differs from the image shown on the exercise sheet, even though I use the homography which is stored in H.mat (instead of my own one).
I attached my result below.