Why is the Lithosphere-Asthenosphere Boundary an Isotherm?
Reading this question, I'm not sure if you realize what an isotherm is - in this case, imagine peeling rock from the surface of the earth until you reached a temperature of 1300 degrees Celsius. Now, in some areas, such as mid ocean ridges, you wouldn't have to peel back much rock at all, perhaps only a few dozen kilometers. Whereas in regions of old, continental crust, you would have to peel off hundreds of kilometers of rock to get down to rock at 1300 degrees centigrade.

The result would be a lumpy sphere at a constant 1300 degrees, which is your isothermal surface or isotherm. The thickness of the lithosphere is very variable.

Interestingly, the Asthenosphere - Mesosphere boundary is created by a mineral phase transition, which is very pressure-dependent; this makes it far more of a smooth sphere.