It's how I always treated brake balance, as a relative number, so I think @
jonjig is right. 5/5 on a ZR1 is not the same as 5/5 on a Leaf! 5/5 simply means 'default'. And it may very well mean that you can make a brake balance of let's say 4/6 and still be front biased!
The theory that I and a few other tuners had is that, since (prior to GT6) you could not change the brakes, the actual bias/force is always the same (rotors/pads/wheel size). The balance is then more like a 'sensitivity' number that allows more control over how (quickly) to apply that force, so it's more of an indirect change of bias rather than a direct one.
And then there is of course ABS, which is more like a 'physics modifier' than just plain ABS, so with ABS on it's even less straightforward.
It could be something like that, although "sensitivity", in my mind, is just "force amplification" anyway. It's not consistently sensible from car to car, because they're effectively different scales per axle, thanks to the "default", built-in bias. But yeah 5,5 is definitely "default" rather than an actual, effective brake bias value - if we knew the built in bias and the zero values, we could still work out the actual bias from those settings, because they are still proportional (as far as I can tell).
As an example off the top of my head, I've been faffing with an R32 GTS25 Type S (NA tuning project; in anticipation of
improved sounds), and the optimal bias I found for circuit driving is 7,10 or 6,9 depending on the track / corner - 7,10 feeling more forward and 6,9 close to the threshold of locking up the rears first (which makes sense from looking at the numbers - 6 in 9 is less than 7 in 10). That's for CS tyres; the stock brakes are obviously very "weak".
With ABS on, though, it has no problem slowing down, but I have to use all of the brake pedal "travel" (same on any car). The moment you turn ABS off, usually the limit of braking power is much lower down in the travel (might be due to excessively forward biases in some cases). ABS is clearly doing its own thing, possibly the pedal travel being a braking power "request", whose maximum is the sum total of available grip, constantly updated from the loads available at the four corners, or something.
The trouble is that the power is directed to the wheels individually according to their load, so the bias front/rear and left/right per axle is controlled by those loads at maximum power, but reducing the applied brake travel changes those biases according to which "channel" gets desaturated first, presumably resulting in equal distribution amongst all four wheels when each is below its limit. Quite how the bias setting fits in with that, I don't know.
Regardless of whether the details of my assumptions there are accurate, what's obvious is that, with ABS on, the four-wheel brake biases are dependent on applied power as well as what the car is doing (and maybe also the setting). Weird.
Anyway, the whole thing implies there are two layers to the bias setting: the stock braking forces
F,
R and the in-game settings
f,
r, which are calibrated on some scale with respective zeros of
c,
d, likely indeterminable. Changing the brakes changes
F,
R, possibly also their relation i.e. the ratio
F/R, the stock bias being
F/(F+R) - call that
B. The in-game setting is effectively a "bias-modifier" dependent on the zeros:
(f+c)/(f+c+r+d) - call that
A.
The overall bias is then
O = AB = [F(f+c)] / [(F+R)(f+c+r+d)], and the effective forces at the wheels are ... in there somewhere.
Nothing like making things simple, is there?
PD: Please let us simply pick values of bias
B and
T total braking force, where the effective forces at the wheels are
F,
R with
F = BT and
R = B'T = T-F.
Simple, intuitive and
consistent! If you fit stickier tyres, just up the force accordingly, rather than juggle single-digit ratios until you find a bias close to what you had before. The brake upgrades should primarily be for fade resistance, since the extra torque from larger rotors can be compensated with a smaller applied force (meaning the same applied pedal pressure for "maximum effort").
They finally caved in with the aero settings, now for the brake bias!
Now if only I have a wheel, things would have been much easier, especially steering the car 
..
