When designing or building, the way to measure is C to C. When buying or selling, the way to measure is C to T. I started riding in 1972 when everything was C to C but somewhere along the way, in the late 70s or early 80s, some marketing genius decided that C to T was a better way to measure. I assume that this was done to allow a customer to gauge standover height, never mind that variability in bottom bracket height invalidates this.