It does, but that doesn't mean your net worth will decrease by that amount that year. There are other effects - e.g. the rest of his investment portfolio.
if you give away $10 then your net worth at that moment goes down...it may easily recover that price, but if you count it like that then you can surely count that the money you donated would have been worth more too
Yeah but nobody has that granular logs of Gates' net worth and donations. At best, it's calendar months, sometimes you can try to assume weeks, with a big maybe.