Clocks used to be able to use the 60Hz cycle to track time, and grid providers would run slightly slow or fast ("time error correction") to get back into sync. A leap second would just be part of this.

I believe in the US this error correction has been discontinued in the East and in Texas, but is still done in the West for some kind of non-clock "inadvertent interchange" reasons I don't understand.

Wait is that why my oven and microwave clocks are constantly getting out of sync by multiple minutes every year?