There is so much to consider. First of all, insulin is not designed to create a perfect BG reading. much comes into play. exercise, food choices, sleep, etc... So, if you want perfect glucose readings, you must try all you can to make perfect choices ourself.

Can't remember the science but exercise has different effect on bs depending on what the level was before starting the exercise.Sometimes bs drops, other times it rises.Try a snack before/ during the session
Length of time spent exercising and intensity also affects it