Because you put TWO (or more) disks at a time on the player so that the first to drop down was side 1, when that finished the next disk dropped down playing side 2. When that had stopped playing you took both albums off the changer and then turned over the pair so that 3 then 4 dropped down - this is an attempt at a graphic explanation
The view on the left shows the disks as you put them on the spindle - the bottom disk drops and its upper surface is read (1) then the next drops and its upper surface (2) is played. Lift thme off and invert the disk stack and you get the right image - and play 3 then four (* mark played side)
With an additional disk in between for sets like Welcome back etc or other triple live albums. I for one, and many others, made sure they were played in the right order, autochanger or not!