At level 50, 31 IVs account for 15.5 stat points, which is then rounded down to 15. But if you then add 4 EVs to a stat, which is 1 point at level 100 and .5 stat points at level 50, you round out the number to 16.
So, at level 50, if you have 31 IVs in defense and special defense, you only...