So it had nothing to do with sales?
In a way, it did. You were more likely to hear the "A" side on the radio and purchase the record based on that. Often the "B" sides were what they called filler, or throw away songs. Sometimes those became the hit side in spite of a record company's push. 45rpm record sleeves in the store, at the time, had the "A" side title listed in larger type than the "B" side.