I was watching Pokemon: Advanced a while ago, and I noticed that when Ash was trying to command Pikachu to use Thunder Bolt on a Vigorath, he said Thunder Bolto. Why did he say this instead of Thunder Bolt?
If he has said this on multiple occasions like you've stated, then it is most likely a mistake in dialogue. Pokemon has been known for its many errors in staying consistent logic or even proper Pokemon moves.
I'm certain he has no reason to pronounce it wrong other than it being a honest mistake.
Hopefully I've Helped!