After ev training

Good question. If when you say "im always off by 20 or even 30 ev points to the maximum stat" that you mean 20 or 30 stat points to the maximum stat. If that is what you meant this brings up IVs or individual values. These are numbers between 1-31 that add that number of each stat of your pokemon. There is no way of really seeing what the exact value of these numbers are, but IV calculators on serebii.net give a good estimation given you know then EVs that your Pokemon has. Sometimes you get lucky and you Pokemon has a 29 or 30 in their best stat other times you don't get lucky and they have a 3 or 4. The IV calculator will ask for the species of your Pokemon, it's nature, level, and what EVs you have given to it. It will do an automatic calculation into a big ugly formula give you a pretty good estimate of the IV values of your pokemon. There is a way of breeding step by step to get higher IV values of the progeny Pokemon but this is a long road and involves lots of luck.
I hope that helped you out at all, if you need more clarification ask another question perhaps more specific to what is troubling you now.