Complex sentence: Here again, various factors suggest great skill on Mozart's part, but not a superhuman miracle: the work in question is somewhat repetitive, and that Mozart was able to return to hear another performance, correcting his earlier errors.
Split into simpler sentences: Here again, various factors suggest great skill on Mozart's part, but not a miracle. The work in question is somewhat repetitive, and Mozart was able to return to hear another performance, correcting his earlier errors.