[antonyt]

In America its pronunciation is closer to "Mastadon." The second syllable sounds like "tuh."

You see the same thing with other words that have "o"s that function similarly. You'll hear Americans turning "o" into "uh" in Tyrann-o-saurus, p-o-tato, etc.

[sph]

But that's not how it's written, is it? Given that English pronunciation rules are very different than how words are written, I would have thought native speakers would learn to actually look at a word and imitate the order of graphemes.

Especially if we're talking about an Internet thing: you've seen it in written form more often than you have heard it enunciated out loud. And seems like the iOS text prediction is also able to spell it the right way.

I don't care honestly, but as a non native English speaker I've never understood why natives make an enormous amount of banal spelling errors.