The happened in December. The statement lands in June. The accounts close in between.
A catalogue earns every day a song is played. But the money is reported in arrears, a streaming quarter ending in December might not be reported and paid until the following June, six months later. A abroad can be slower still.
That gap is the reporting lag, and it creates a genuine problem at year-end. The income was earned in the period you're closing. The cash, and even the statement that tells you the exact figure, won't exist until months after the books are signed.
So the reporting question is not 'how much cash came in?' It's: how much did this catalogue earn in the period, even though no one has told us yet? Wait for the statement and you understate the year. Recognise the earned income now and you're making an estimate, one you have to be able to defend.