out_ds logarithmic/hybrid volume setting

This is about the settings linear, logarithmic and hybrid in the DirectSound output plugin configuration window. Now, I know what these settings do, so that's not the problem.
The question is, why do the logarithmic and hybrid settings cause the volume to be -3 dB when it's set to 100% and are you only able to reach 0 dB on the linear setting? That's what the "Status" tab reports, as well as the peak meters of my sound card driver.
This has nothing to do with the map 0% to xx dB setting.

Can anyone give me an explantion for this please?