Got a Nest for Christmas and I have the same problem. I could deal with it if it was a constant offset, but it's not. Some times of the day it will read the same as 2 other thermostats, other times it's a couple degrees off. One night it ended up being 74 (Nest set at 70) in the middle of the night, the next night it was 66 (Nest set at 69).
Sounds like it's a major design issue. Considering my options now, since it was a gift and no gift receipt I'm not sure if I can exchange for a Honeywell and where.