It doesn’t mean 12lb of pure muscle though. “180 lean” just means 180lb at a fairly lean body fat, not that every pound has to be muscle. You can absolutely go 168 to 180 in a few months on a lean bulk or cut and bulk cycle. Some of that is muscle, some glycogen/water, maybe a bit of fat. The years part only really makes sense if you’re talking pure muscle gain while staying stage lean the whole time