Max is right on the money with his reply. The bias is just a modifier to the "standard" brake distribution, which we can't see directly in game, but will be front-heavy.
When using bias (also in GTSport), the very top of the braking bar goes red (I checked this with no ABS). I take this to mean the overall braking force is weakened because you are creating the bias by weakening force on one axle. So if you adjust bias rearwards, the front brake gets weaker (compared to its strong starting value) and the rear stays standard (at its starting value), you do not get a better rear brake. But the effect is still "bias rearwards".
You can see that if you change brake kit and brake pads, using no ABS: if you can just lock the front wheels, then adjusting bias rearwards will make the front wheels un-lockable as you've weakened the front brake. So in this case you would then need to upgrade the pads by one or two stages. I generally tune balance without ABS as you can see which wheels lock and when. You can then re-enable ABS when you're finished.
The red section on the braking bar seems smaller than what I remember in GTSport. In GTSport, you could look at the +5 bias red section, and the -5 red section, and they were different sizes. It's probably possible to reverse engineer the hidden "standard" brake distribution from this. I have not (yet) tried this in GT7...