I've never been able to fully understand the 1 point drop and dc colour certainty either.
From my understanding the 1 point drop is based on the assumption we have 3ppm of co2 in the degassed water, so a ten fold drop (1ph) equates to 30 ppm co2. However what if the degassed water only contains say 2 ppm of co2, that would only give us a 20 ppm reading, and maybe not the perfect lime green dc colour.
If the above theory is true, and I DON'T KOW if it is, then it might explain why some people need larger ph drops to achieve a nice lime green drop checker.
That's my random Saturday morning thought.