Theorhetically, each 'model' of thorlo is sewn to function properly based on the type of motion commonly associated with the activity for which they are marketed.
I've gotten the walking, fitness walking, running, and I think light running thorlos, and compared to my old nasty all cotton socks, they all perform magnificently. However, there is a significant difference between the walking and running socks...the running seem to have less padding in the non-impact areas. I'm guessing the ergomonics of walking require more all around padding, plus the fact that one typically doesn't 'run' all day, whereas generally you can walk till you drop.
How the walking differs from the tennis I don't know, but tennis has alot more lateral movement, with lots of hard stop and go's. My guess is the sock is padded for that more than you'd find in the walking sock.
The 'walking' sock I have comes in both white and black. That's great so I can wear them in my casual life (white) and in my enforced business attire life (black).
They also have different ankle heights, but I'm old school. I think socks should at least cover the ankles and shorts should be well above the knee...not socks barely out of the shoe and shorts 1" less than 'floods'.
-Joe
Aspiring curmudgeon who likes his shorts short and his athletic footwear called sneakers.