Here is magikarps pokedex entry for Pokemon heartgold and gold versions:
An underpowered, pathetic Pokémon. It may jump high on rare occasions, but usually not more than seven feet.
Ok, yeah that's reasonable. Now let's take a look at the black/white entries:
A Magikarp living for many years can leap a mountain using Splash. The move remains useless, though.
Leap a mountain?! Well, duh, there is a big difference between 7 feet and a whole mountain! Furthermore, jumping over something that huge is practically impossible!