I do exactly the same cardio exercise three times a week. Exactly the same, almost to the second. (I'm somewhat red-faced to admit that it is a Gilad video from 1992, doing step aerobics). I noticed that on some occasions, my heart rate would be ~30 bpm lower at maximum, and there would be almost no time in Zone 5. The cause? Metoprolol. If you're on beta-blockers and don't take them at the same time every day, there will be a change in your workout heart rate. There are some natural things that have similar effects to beta-blockers, but they do not affect adrenaline. I don't know if any foods or supplements would affect your workout heart rate.

This is what I saw when I experimented with varying the time from the last dose (metoprolol's half-life is, meh, about 6-8 hours for me). The "with metoprolol" dose was taken at 7 am. The "without metoprolol" was a skipped dose, and varied the prior evening's dose time. I was shooting for a 10-hour and 20-hour interval, but I often can't quite get my great big giant fat ass into gear.