Nick,
How do you verify the accuracy of your meter and what is your calibration process. Unless the meter you purchased is a high end meter your statement that it is very accurate is at odds with all that I have been able to research. The repeatability and accuracy of a dB meter is, at present, directly proportional to price. The lower end Radio Shack meters are incapable of accurately measuring sound pressure levels (SPL) in a repeatable manner.

Anyone wishing to be able to accurately measure SPL for the purposes of comparing noise levels from different sources on different days is going to have to spend $400 to $1000 on a meter and about the same amount for a calibration attachment. Unfortunately, there are no cheap sound level meters that are repeatable and accurate.