It's a fairly common trend to mispronounce the name of a Pokémon. I would say the reasoning is a combination of never actually hearing the name pronounced outside of the anime (and even then I would never expect some of the pronunciations on there to be correct) and that some have origins based in languages other than your native one. A lot of times, it really isn't easy to get it right, so we just make up our own pronunciations. Which names are you unsure you've pronounced correctly and which names have you discovered you most definitely have said wrong?
You can even say why you've pronounced names the wrong way. Maybe you just attach very literal pronunciations in relation to the origin of the word like me or maybe you didn't catch name's origin.
This can also be applied to names of places or characters, etc.
Here are a few of mine:
Garbodor- I used to pronounce it GAR-ba-dor. Never really caught the "odor" part until I watched the anime.
Unova- Always said yu-NO-va, myself. Again, the anime proved me wrong.