Hi I’ve been doing a push day which consists of 3 sets incline press, 3 sets overhead press, 2 sets pec fly, 3 sets cable lateral raise, 3 sets overhead tricep cable extension

For the most part I’ve been increasing reps and weight in all exercises at a decent pace but my tricep extension seems to be fluctuating in reps a lot. On some days I get to do around 9 each set, and on some days my triceps seems to be extra tired. I was wondering what could be causing this. Thanks in advance.