1 stat increase (at LV100) = 4 EV points, but then when you put 252 EV's in 2 stats, you get left over with 6 EV points...
So why waste it when you can put it into a third stat?
(Obviously when I mean 6 left over, only 4 points is actually useful for the Pokemon. The leftover 2 points does nothing to the stat.)