Not that I'm defending them, but I know when I got the two mixed up it wasn't that I didn't know the difference, it was that I couldn't spell definitely and spellcheck always came up with defiantly and while I would go "I'm pretty sure that's not how it's spelled......" I couldn't make it come up with anything better until I tried the actual paper dictionary.

So...just throwing that out there. I find myself chuckling when I see it and wonder how long the author fought with their spell check before giving up and going "FINE! maybe that IS how you spell it!"