Why did my last few sets earn less XP?

The more sets you pile onto one muscle in one workout, the less each extra set earns. Your first sets on a muscle pay full. Later sets on that same muscle pay less and less.

This counts sets per muscle, not sets in the workout. Moving on to a different muscle starts that muscle fresh.

It also resets every workout. Your next session starts every muscle back at full, however much you did today.

Nothing is taken away here. A late set still earns XP, and the XP you have already earned is never removed.

Learn more: How do I earn XP?