Well im not sure, but SSS chart is guidline, most of time you shold be ahead if You follow it.
Example: You are playing HU and both players commited 1$ to the pot and both of players push preflop. One of players have pocket AA and other one has pocket KK. Both are strong pockets and are to be shoved if someone rises preflop.
KK guy will be minus All-in EV in HEM math sense, however this is cooler, cause both guys had good pockets and were expected to push in SSS preflop.
So All-In EV shows how correctly You guessed the villains hand range before pushing in. If You correctly estimated that he IS BEHIND You get + if not, You get -. So if the guys VPIP is like 2% and PFR is 0 and hand history is big, and guy shoves preflop, that means he has something like KK or AA, this is trivial hand range reading example. You should then think twice before pushing with something worse than KK.
If You shove in flop, turn, river, then math goes more complicated cause of complicated multiway pot structure, but main idea is the same.
Sometimes playing by SSS charts can lead You to - All-in EV. However thats variance. 
Hope i did not mess up, just learning!