It's the order they joined the band. John started a band called the Quarrymen which Paul joined a year later and George joined a year after that, when Lennon's Mother died the Quarrymen fell apart and resurrected a year or two later as the Beatles with Pete Best on Drums(it's a little more complicated than that but whatever). When they Went to record their first single, producer George Martin decided Best wasn't a very good drummer and wanted to use a studio musician on the record, instead the band decided to just replace him entirely with Ringo.
__________________
|