the final number of the stat is not simply +1 for every 4 ev, it's only fit for 100lv
I can't find the formula in english wiki (bulbapedia), but in the japanese wiki
https://wiki.ポケモン.com/wiki/%E5%8A%AA%E5%8A%9B%E5%80%A4
努力値に依存して0から63までの努力レベルが存在する。努力レベル=int((√(努力値-1)+1)/4)
in english, ev level is within 0-63, ev level = int((√(ev-1)+1)/4)
"ev level" is the one which actually adding into stat, ev does not
thats why ev in low level poke doesn't affect like in 100lv poke