1. Typing uppercase characters counts as a mistake
I'm not sure how that got to be the case, but somehow typing an uppercase letter instead of the lowercase is a mistake, despite the fact that sentences start with a lowercase letter. This conflicts with my muscle memory of starting sentences with a capital letter.
2. WPM is not a useful metric on its own
WPM can rise and fall depending on the length of the word. The bigger the word the less likely you are to type that word correctly from muscle memory, so the speed drops. The speed also drops due to the word being longer. I believe having both metrics would yield more useful data, such as when do you slow down etc.
Speaking of which, there are some more statistic things that could help, like measuring how fast you are at fixing the mistakes, or measuring three-letter combinations instead of two-letter combinations, because the context of the third letter might help, but you do need more data to gain a statistically significant result. Maybe trying to classify mistakes by the side of keyboard they happen on -- i.e. are they simple typos or a miscoordination of your hands.
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Also, as pointed out by another commenter, hands also threw me off. I've been observing them and it's interesting that I don't use my little finger for the left row -- it's used in case I need to press shift.