It's not nearly as simple as it sounds in theory. Consider that you have typos which are from hitting the wrong button - and also typos from hitting the next letter before the previous letter.
Also when looking at typos it's likely you'd find statistically significant differences for typos by hitting a letter to the left, or right, or prob. of typoing given the letters general position within a layout etc.
Doesn't any spellchecker use keyboard layout-aware Damerau–Levenshtein distance metrics ? https://en.wikipedia.org/wiki/Damerau%E2%80%93Levenshtein_di... doesn't mention keyboard layout-awareness variants...