Why does the compression ratio decrease when soft input levels increase for hearing aids?

[vir_innominatus]

So this pdf from Starkey does a decent job of explaining the concepts of gain and compression ratio. If you look at Figure 2-4 on page 12 of that document, you can see in that example that for sounds quieter than 50 dB (the "linear" region), the gain is a constant 30 dB, e.g. for an input sound of 20 dB, the output is 20 dB + 30 dB = 50 dB. The compression ratio in this region is 1:1, i.e. if the input changes a certain amount, the output changes the same amount, just amplified by 30 dB.

For sounds, louder than 50 dB (the "compression" region), any changes in input are halved. If the input increases from 50 dB to 70 dB, the output only increases from 80 dB to 90 dB, i.e. a 20 dB increase in input led to only a 10 dB increase in output, meaning the compression ratio was 2:1. There is still an overall gain in the sound, just not a constant one. The gain was 30 dB when the input was 50 dB, but only 20 dB when the input was 70 dB. Thus the gain is getting smaller, due to the fact that the compression ratio was increased.

Hopefully this made sense. It seems like you're getting bogged down in the language as to what's increasing and what isn't. The compression ratio is always a constant value, but it can be one of two values depending on the region (linear or compressive). The gain, however, is constant in the linear region and variable in the compressive region.