I said that a 100gr 6mm will have a higher BC than a 100gr 6.5mm. which is true if you compare VLD profile bullets.
BC is sectional density with a form factor for the shape of the bullet thrown in. So, yes, I'm not surprised that a heavy-for-caliber 6mm 100 grain bullet has a better BC than a light-for-caliber 100 grain 6.5mm bullet.
The problem is that it's a distorted comparison, for the same reasons listed above in the discussion about sectional density.