Anecdotally, nutrition labels are often mathematically calculated based on the underlying ingredients and often are not measured or tested for under laboratory conditions. When they are tested for, the variation appears to be substantial. The reason is that testing is expensive, and as it's often not required, there's little reason to do so. It's almost completely voluntary, and there does not seem to be any significant repercussions for false-statements — at least those made non-maliciously.