TPS54560: θJA < θJc(top)

That means the thermal resistance from junction to case top is larger than the combined thermal resistance from juntion to ambient.

You can see the θJc(btm) is much lower than θJc(top).

θJA means if you have 1W power dissipation, the difference of the junction temperature and ambient temp is 42C. θJA includes all thermal dissipation path from junction to ambient. Path from junction to case bottom to board to ambient is parallel with path from junction to case top to ambient. Just like current will tend to go through least resistance path, majority of the heat will go through the easier path, which is through case bottom, since this package has a thermal pad. Only very small amount of heat will dissipate through case top.

θJc(top) assumes all 1W heat will disspate through case top, then the temperature difference from junction to case top will be 45.8C.

Hope this helps.

-Yang