6600M dBm vs S meter

I am trying to get myself straight on signal levels -dBm and S-units. In particular I'm concerned this evening about 80m.
I have set the rig to CW with all noise reductions off and a bandwidth of 500Hz.

With no antenna connected and 0dB preamp gain, I have:
-130dBm and S2 on the S meter. My understanding is that -130dbM should S0.

With the antenna connected and 0dB preamp gain, I have:
-108dBm and S6 on the S meter. My understanding is that -108dBm should be S3.

Any input is appreciated.
73,
Fred
AE4ED