As mentioned in the comments, I got one of those high-level Magikarps from the resort in Platinum and evolved it into Gyarados. I did a bunch of EV training and its attack went up 28 points I think it was.
The reason: when you get EVs, they are not all added at once, they are added gradually as you level up. You normally get most of them before level 100.
For example, if you got 100 EVs at level 1 (actually impossible but just assume) then you would probably only get one extra point per level (on top of the normal points you gain anyway).
If you were level 90 and got 100 EVs in that level, you might go up 30+ in one level. If I have the patience I might try it one day, you can get really high level wild Magikarps in Platinum.