Hi, Jo.
Without knowing any of the details of the corporation involved, I can't be specific, of course. But many accounting determinations cannot be made until the corporation's year-end. Estimates are recorded periodically during the year and, sometimes, dividends are declared based on these estimates. When the books are closed after the year-end, the final calculations are made and it just might turn out that the profits were less than the dividends that had been paid out. If the company paid out $1 million in dividends, but only $900,000 profits were realized, then where did the other $100,000 come from? Must have been paid from borrowings or from capital that had been previously paid in by stockholders, rather than from earnings of the company. (In a regular corporation, the dividends might have been paid from prior years' accumulated earnings, but mutual funds are not supposed to have any earnings carried over from year to year; they are supposed to pay out everything each year.)
At any rate, some of the dividends that have already been paid and reported preliminarily as having come from profits, must have come from some source other than profits, that is, a return of capital. So an adjustment must be made, and it is very difficult, perhaps impossible, to assign it to any specific monthly dividend during the year. It's just an adjustment to the year's total dividends. Sometimes a determination is made that a prior year's profit was miscalculated and amended reports are issued - and the shareholders must file amended returns.
Since the shareholder is taxed only on the fund's profits, these additional amounts are not currently taxable to him/her, but reduces his/her basis in the shares held. When the shares are eventually sold, the gain will be larger because of the reduced basis. If the return of capital is so large that it exceeds the basis, then all such amounts received after the basis is reduced to zero are taxable gains.
I hope this "generic" explanation helps a little. But remember: I've been retired a long time. Check with your own CPA to be sure of the current rulings and accounting treatments.
RC