I don't think it's actually a different meow. I think they are just more vocal and use more of a range with their meows. They just have a lot to say, LOL. Think about it. Most cats only meow once in a awhile so if all you ever said was "Hello" then you wouldn't really change how you said it. But you talk far more then that and have a wide range of tones you use with your words to help get your point across. Same with Siamese cats(or more vocal cats in general).
That's my theory at least, haha.