Because you're lying? Consider this, I've determined, using the analytical balance I have here, for the same sample (the O.P. sent it to me via TCP/IP) the actual mass is 8.995572 g. Aren't I awesome?
No, I'm not. You know I'm lying, you know the outcome described is false. There is, IMHO, no distinction between reporting accuracy you're not sure of, reporting accuracy that isn't possible, or even reporting a measurement you didn't make.
One time, the maintenance crew came into the lab, there was something wrong with some part of the facility, and they wanted to compare their handheld pH meter or check it, or something. Its hard to know what they wanted ...
Anyway, we compared their grimy handheld pH meter, with our validated, weekly maintained pH probe. I carefully preformed a two-point flanking calibration with NIST-traceable buffers, cracking open a fresh bottle to be extra sure. Our pH meter gave a different result than his, and he blew up at us -- his was obviously better -- it cost $80 and gave the answer to 3 decimal places, and ours stopped at 2. He just arbitrarily decided that more decimal places = better. And that's not true.