if I'm using a comparator to measure CBTO, I am still using the same reference of distance between ogive and lands.
You aren't really measuring what you think are with the tool. The ogive is the entire arc from tip to the bearing surface, not a specific point on that curve. So like Bob said, what we're really doing with the comparator is measuring from an arbitrary point on the ogive to the base of the cartridge.
The distance between the measurement point on the ogive and the actual beginning of the bearing surface is the same (within manufacturing tolerances) for bullets of the same type, but not across different weights/makes/models of bullets.
My 30 cal comparator insert measures between .280 and .293", neither of which are to the .300" bore or .308 grove measurements of a 30 cal barrel that the bearing surface of a bullet would actually engage with. So my CBTO measurements are based on how the oblong opening particular to my comparator insert sits on a particular bullet. The Hornady tool isn't exceptionally high quality either, I'd put money down that we would get fairly different measurements using two different inserts on the same bullets. Meaning my numbers only matter to me with my particular insert, and everyone else's would be different because of tolerances in the comparator.
If we could standardize at what point along the ogive to measure at, and get tools of high enough accuracy to measure it consistently, the CBTO itself could be a relevant data point. But what it's really doing now is describing the relative positions of several data points of a particular bullet in relation to the case.
Berger is awesome because they publish detailed information on the various measurements of their bullets. They've standardized the BTO measurement as the point at which the nose contacts the rifling. But my comparator isn't .300, so I can't measure exactly what they do unless I find a source for a (probably really expensive) precision version of a $30 Hornady tool that works fine for any given bullet. It just doesn't have much comparative value across multiple bullets of different types.
And even 0.300" for a comparator really isn't correct either; I don't have a print of the reamer used for my chamber, and with the number of rounds through these rifles who really knows where my lands truly start? And they move as you shoot. The tolerance stack here is to the point it's easier to just measure the gun for each different bullet, call that measurement a relative description of the bullet's relationship to the case and a potential window of jumps, and load on.
