Well, it does say "less than" -76 dBm, so maybe it's a lot les than...
Andy's idea that Yamaha may specify unweighted instead of A- (C-, whatever) weighted makes some sense.
As for your earlier question, assuming S/N does not include distortion components, and given they are using dBm for output as well, then the S/N ratio is >100 dB from max output.
I am surprised Yamaha chose dBm, not sure I have seen that in audio (or have just forgotten) -- except in tuners, which sometimes used dBm or dBf. The pro equipment I recall off-hand all used dBu or dBV. As Andy said, in the RF world we use dBm all the time, and power transmitters use dBW, referenced to either 50 ohms (most RF) or 75 ohms (video systems). (And dBc, dBHz, dBi, dBK, dBmV, dBZ, etc. etc. etc.) The 600-ohm reference comes from Bell telephone days (the B actually stands for "bel", a unit named in honor of Alexander Graham Bell, but a bel is just too big for most practical use so the standard is decibels, dB, one-tenth of a bel).