Pre-digital car radios had smaller dials - usually a horizontal strip with short digits - 55 - 7 - 9 - 11, etc. In those days, most AM stations shortened their frequencies for branding purposes if they were at the left half of the dial (so to speak), but not if they were on the right side, and above 1000.
In LA there was 57/KLAC, 64/KFI, 93/KHJ, and KFWB "Channel 98," later "News 98." But over 1000, the stations generally used their whole frequency - KGBS/1020, KNX/1070, KRLA/1110, KIIS/1150. and so on.
Another exception seemed to be frequencies that ended in 10. So KMPC went with "710" not "71." I always figured it was because it took less syllables for the DJs to say "seven-ten" than "seventy-one."
Same with 610/KFRC in San Francisco. All the other Bill Drake ("Boss Radio" stations) were shortened, like 93/KHJ, The Big 8/CKLW - even 136/KGB.