It's really an approximation. It's not really 12000, but actually 12470. Actually the 1.73 factor will put you real close.
Also you have to understand that a lot of folks refer to it in just a close number. Used to always call it twelve five when it's actually 12470. The 4 KV most people mention is actually 4160 phase to phase. Worked a lot of 34.5 KV and we always referred to it as 19.8 phase to ground but I think the pots label says 19920 or something like that. I've heard lots of people mention 24 KV or 25 KV when actually it's 24.9 KV or thereabouts. Common REC voltage anymore and some utilities use it. If you'll look at the pot labels ever time you'll see.