I have noticed this too and after some experimenting I think I have found the cause.
If you have extra spaces between words those spaces will get counted as words. It’s easy to overlook as when editing using the visual editor the hidden spaces are not shown, you have to go into the html editor to see them.
Clearly it’s a wordpress bug as it should ignore the extra spaces, but at least there is a work around, albeit a rather tedious post editing one.
