[Promit]

You don't want to encode brightness perceptually until you're showing it to the user. You need a linear space to actually do lighting calculations, which means physical luminance values. Within that space, you can use whatever scale you wish, with consequences to banding and quantization artifacts for decimating the bit depth. Tone mapping includes conversion to a log space via the gamma curve.

I explained it in another comment but basically the "20 bit" value is based on an idealized digital image sensor rather than a game rendering pipeline (which operates in floating point). It has admittedly proven somewhat confusing.

[Dylan16807]

It's worth noting explicitly that floating point numbers let you have a linear scale but logarithmic-ish storage at the same time. A 12-bit float could represent a 1:1000000 range, let you perform normal linear math, and also be just as banding-free as a 20 bit integer.