The mug on the left was in a hotter part of the kiln (gas reduction), it's surface is brilliant glassy smooth and metallic. The one on the right is dull, pebbly, much less interesting. The temperature difference is about one cone. This is not enough to make much difference in the transparent glaze, but the tenmoku is sensitive, it needs to reach the full temperature.
Tenmoku is a kind of ceramic glaze. Glossy, very dark brown or maroon, edges crystalizing, firing at high temperature in reduction atmospheres.