You think you did, but you clearly didn't. My point was that your HVAC system (your furnace and A/C combined) AS A WHOLE is not capable of maintaining such a narrow temperature range. The Nest, or any thermostat, can only work with the tools it's been given. What you're expecting is equivalent to giving an engineer a yard stick and then asking him to measure something to the nearest micrometer.
Some people just believe what they want to believe, and no amount of reason will persuade them.
Of course they are. I manage it manually in that exact temperature range. If I can do it, so can NEST.