This was a process known as "automatic sequence."  It was common practice to put Sides 1 and 3 on the first album and Sides 2 and 4 on the second album of a double album in the 70's, especially for live albums.  Notable exceptions to this were KISS Alive! ans KISS Alive II, the latter being that only three sides were live anyway.  In contrast, Bob Seger's Silver Bullet was pressed in automatic sequence and even the three-album Woodstock compilation went so far as to have Sides sequenced as 1 and 4, 2 and 5, and 3 and 6.  The pressing of THIS album in such fashion was certainly not in error.