cus u "paid" a stock dividend, which is not really a cash payment, but rather increases your stock outstanding by 20%. and the rule is if u issue stock dividends during a year, then the outstanding after stock dividends should be dated back to the beginning of the year. (also ignore the original standing before) so effectively, for weighted average, u count the "split" stock outstanding for a whole year, and the buyback for a quarter.
in short, deal with a stock dividend same way as a split, and a stock dividend is totally different from a dividend (of a stock).

stock dividend 20%