It means that they have already filled out 100 Ev's in that Stat.
Calcium Protein and Other Pokemon Viatamins all increase Ev's by 10 per use.
Now, You can't go over 100 Ev's using Viatamins, so if you already have 100 Ev's in a stat from battling other pokemon, than your Viatamins won't work in that stat.
Or if you've already maxed your Ev's to 510.
http://pokemondb.net/ev Should tell you anything you need to know about Ev's.