Lotta good points mentioned; beginner gains, different genetics, diet, but even just looking at your differing training approaches you realize that you're both employing different methods of overload.
Your focus on peak contractions is (hopefully) trying to coax the extra intensity out of the isometric portion of the ROM. Your friend, alternatively, is making use of explosive concentrics, which can create the perception of more stress (more 'weight') on the muscle as it accelerates the resistance. Both are certainly excellent methods of training. Not to say that one if superior to the other, but maybe you might be better served trying to make use of the other approach if you feel it is what is creating the different levels of progress.