Heat capacity?

This is because of the high specific capacity of pottery compared with that of metal. For example, the specific heat capacity for porcelain is 1.07 J/g-K, whereas that for aluminium is 0.87 J/g-K, and that for iron is 0.46 J/g-K

Because of the high specific heat capacity of pottery, more heat is needed for the pottery bowl to have its temperature rises by 1'C than it is required by the metal bowl. The equilibrium temperature of the soup in the pottery bowl is thus lower than that in the metal bowl. Therefore, more heat has been transferred to the pottery bowl.

Since more heat is transfered to the pottery bowl from the soup than it is in the metal bowl, the temperature of soup in the pottery bowl appears lower.