If you have an odd IV number like 31, you will get a stat increase with 252 Df EVs.
A little example: Furfrou has 60 Base Def. On level 50 : ((60*2+31+252/4)/2+5)=112. He would have 112 Def on level 50. If you change to 248 EVs, he would have 111.5 Def. Pokemon only calculates integers and that...