It sounds like they can't figure out where all of the heat is coming from. Is it possible this moon is partially heated by radioactive decay like the earth is?
Enceladus has a density of 1.6 g/cc, so it is mostly water and has little of the heavy radioactive elements. It probably does have a noticeable amount of light potassium-40 though. Earth has a density of 5.5 g/cc, because it is loaded with the dense elements, especially the uranium and thorium that make much of Earth's geothermal heat.
http://en.wikipedia.org/wiki/Geothermal_gradient