AirForbes1

I wasn't sure if I should post this in the question forum or this one. Anyway, I have a question about B sides and chart listings. Were chart listings determined by sales? How would they determine the listing of B sides? Paperback Writer/Rain was a single issued by The Beatles. Paperback Writer, the A side, hit #1 in the UK and US. Rain went to #23 in US. How did they determine that Paperback Writer went to #1 but Rain didn't even make the top twenty? Thanks.