FWIW. The upstream power is fine. We've had posters here with 28 dB of upstream power and their internet works fine.
It may just be a very clean, low attenuation, and noise free return path. It may be correct for your local system specs. There are spec variances between individual systems.
Putting an attenuator in the line will only make the upstream power as read at the modem look prettier. It doesn't really do anything. The power is controlled automatically by commands from the cable modem termination signal at the local headend facility.