Without getting too far into the specifics of it, the db meter is correct. If you've turned the volume fader down on the kick's Mixer track, the -10db you're getting is just the read-out from the Mixer track. The actual signal going into the Mixer is still -6db, and this is really, really important to remember for all audio going into the Mixer.

If you have a channel, sound, synth, etc, that is already clipping the output BEFORE it gets to the Mixer, lowering the volume fader in the Mixer will NOT prevent that clipping from happening, because it's not actually lowering the original signal - just the mixer volume. The initial volume of a sound before it hits the Mixer stays the same. The Mixer just acts as a gain stage so you can then balance that sound with others in the mix.

This is really important to remember when designing synth sounds. If you clip the audio in the plugin, it's not going to go away just by turning down a Mixer or Channel knob - it has to be treated at the initial source first.

Depends on whether or not two things, the audio engine and the plugins used, make used of floating-point. Audio engines any any modern DAW will usually use at LEAST 32-bit float (more likely 48 or 64-bit) so we can more or less ignore that and focus on the plugins.

Assuming both your DAW and all your plugins use floating point, then the following scenario would be true:

A channel is 'clipping' at +10, so your master channel is also clipping by dB. You bring your master fader down by 10dB and now your output peaks at 0dB, problem solved.

Now take the same scenario but put a plugin that only uses 24-bit onto the channel that is clipping. Both your channel and your master would read 0dB and be hard clipping, dropping the master 10dB would make it read -10dB because it's been clipped before it got to the master.

Floating point basically let's you go above 0dB internally as long as it's dropped back below 0 before the final output, but not all plugins work that way so it IS possible to clip the channels if you're not careful.

So, wait... FL doesn't use 32 bit FP internal mixing?

FL does indeed use 32bit floating point for the internal Mixer tracks, so they can be pushed beyond 0db, but the true gain stage of sounds in FL Studio depend on the audio source going into the Mixer first. So, in this case, the Kick was reading at -6db before the Mixer track, and that would not change to -10db unless the Kick itself was lowered to -10db before being sent.

There is another side to this however: the Mixer volume faders in FL Studio are set AFTER the track's FX Chain in the audio signal path. So, to further complicate it all, the end db level of the Kick's track is still -10db - that's what's going to the Master track in the end - but the db meter in the FX chain is correct in reading -6db, because it comes before the fader adjustment.

In a nutshell: Source sound in Channel Rack (Kick) volume > Mixer track FX Rack > Mixer faders and controls (panning, stereo) > Master track. That's FL's gain staging path anyhow.