I guess it's just you; lol (by the way-it's "Definitely" , no "a"). I think what happens is you get so into what you are reading, you don't notice the odd spelling, but just accept it and rad on, but you brain retains it and pops it out next time you write something. For me, I always notice the olde tyme spelling, think "how weird", then move on.
But you probably retain more of the knowledge from what you read than I do. Most Americans don't spell that well anyway - they keep dropping their "U". Colour, Flavour, doughnut, etc. :-)