Why did one muscle earn more XP than another?

Every set splits its XP across the muscles it trains. Most of it goes to the main muscle. The rest is spread over the muscles that helped.

A bench press set puts most of its XP on your chest, and a little on your triceps and shoulders. So a workout full of pressing leaves your chest far ahead of your triceps.

A muscle at a higher Level also earns a bit more from the same work. Two muscles in one workout can end the day apart for both reasons.

A muscle you did not train earns nothing, however hard the workout was.

Learn more: What does levelling a muscle give me?