Some Onkyos have a user configuration to allow you to set the maximum volume to be less than the actual volume. That way you don't blow your speakers by mistake. If you bought it used, it is possible this setting was used by the previous owner. It was my impression most Onkyos go 0-100.
Other than that, I usually listen to my Onkyo at -30db to -15db, which probably translates to around 55-80 on the volume dial, however the volume #s are not necessarily the same across receivers.
If you can turn the volume high enough for your tastes, without distortion, I wouldn't worry about what the volume indicator says.