I think the author tries to demonstrate the different levels of rank in the game and how the usage of hyper and bombs can increase or decrease it. +0 being the regular rank, it increases throughout the game as it can be seen in the fast-forward replays of stage 1 (+1, +2, +3). A hyper increases it (stage 2 boss: +3 -> +4) and a bomb will lower it (+4 -> +1). Interesting to watch!