Probably due to calibrated altimeter. If it was set a couple hundred feet off to account for what is known in aviation as Density Altitude.
I wouldn't say it is notable as it pertains to the bread itself, but is a possible explanation for the negative numbers. If the air is hotter, it is thinner, and colder it is denser. In occasional conditions, pilots have to adjust the mean position of the altimeter instrumentation because the pitot tube or sensor may pick up air pressure that tells the plane it is flying at a different altitude than it actually is.
Here is an FAA doc for sauce. Hope its helpful.