The difference between looking at Ks after a season and looking at Ks before a season?
The whole point is that if two guys have the same triple slash, the same 2B and HR, the same SB and CS, but one guy Ked 60 times and one guy Ked 180 times, they were basically equivalent on offense. I'd take the low K guy, but it's pretty marginal. But if 180 K guy goes out and cuts his Ks down to 100 the next season, odds are he'll be better than he was the year before. Not every time, certainly, but more often than not.
You don't seem to grasp the difference.