fast typing?
I'll just observe that the "a" and "i" are quite far apart on the keyboard. The "a" is pressed by the little finger of the left hand, and is on the "home" row, while the "i" is pressed by the middle ("the" finger") of the right hand, and is on the row above the home row. Kind of a hard typo to make.
However, I guess certain dialects of American English, including mine now that I think about it, might have that first "i" sounding somewhat like a short "a", and thus the error. I've seen it done consistently by some posters, not necessarily you, both in multiple uses in the same post, as well as across posts and even across threads.
However, the spell checker is our friend. It most assuredly is mine. I used it at least 3 times as I composed this reply :)