OT, but... When and why did people start writing "70ties" and "80ties" instead of "70s" and "80s"? The "ty" sound is already in the number 70 so under what logi

OT, but... When and why did people start writing "70ties" and "80ties" instead of "70s" and "80s"? The "ty" sound is already in the number 70 so under what logic does it make sense to talk about the "seventyties" and the "eightyties"? Or is it just a hipster tic to make me, who was alive in both those decades, feel like an old fart?