Think of what happens to a normal roof tile in the sun: it absorbs solar energy as heat and also reflects some and radiates some away. A solar panel is the same

Think of what happens to a normal roof tile in the sun: it absorbs solar energy as heat and also reflects some and radiates some away. A solar panel is the same (though a bit more reflective), but when a load is connected some of the solar energy is converted to current instead of being absorbed as heat. Therefore the panel is a little cooler when a load is applied.