But then you get the (in some ways correct) result that the difference between the 15th of one month and the 15th of another month is a couple months and couple of days, instead of the intuitive whole number of months.
That gives you an absolute time result, not a practical one. Using that methodology the calculator wouldn't be able to tell whether something is in one month or not.
The Windows calculator is terrible design. To calculate deltas between two dates you need to convert them to seconds, subtract them, and convert the result back to an absolute time.