The meter readings on the 7600 are relative to its peak level (about +25dBm). So, a reading of -12 on the 7600 means about +13dBm. There is nothing wrong with your 7600 – your problem is that you are confusing the different indications.
dB is always relative to something else. What this is depends on context.
dBm is relative to 0dB being defined as 0.776Vrms into 600 ohms.
dBu (and dBv) is relative to 0dB being defined as 1.0Vrms into 600 ohms.
So, you really have to know the context before making these comparisons.
Can't really add anything else.