Nikon D300 iso L1.0

Some strange thing are happening with my D300 files. I check the exposure with handheld lightmeter, over my 18% grey calibration card. Shooting with these settings gives me approx. 0.3 underexposed pics. Nothing wrong with that, I just have to recalibrate my lightmeter. The strange thing is that if i shoot with iso L1.0 (100), the pics are 0.65 overexposed, in comparision with iso 200 or iso 400 (Measured in C1 V4).
Is that a software bug?