Sounds like a calibration issue, presuming there aren't other factors like the angle you are using with the spotmeter. Transmissive loss is another factor, but I'd expect that to be within 1/3 stop with modern lenses. Doesn't the booster have a calibration dial on it? I haven't used the Sinar Booster I, but I have a Minolta Booster for the Flashmeter series, and I thought most of these had a gain control on the booster.