Single Band compressor no longer fully compressing?

All amplitude-based effects in Audition are referenced to 0dB, so to use one correctly, your signal has to be normalized to 0dB first. This is easy; you go to Effects>Amplitude and Compression>Normalize and select either 100% or 0dB (they are the same thing, but only at that level - the more useful dB scale is logarithmic and quite frankly I have no idea why the percentage scaling is even there at all - it doesn't relate at all to anything your ears will tell you)

This will have the effect of making the highest peak in your recording reach 0dB. It won't alter the sound - except to make it a little louder in your case, but it will mean that the settings you make will actually mean something. Yes the effect can be made to work with lower levels of audio, but if you don't actually know what your peak level is, then you'll never be able to set the controls accurately.

In your example above though, the reason your signal goes above -9dB is easy - you've set the threshold at that value, but the ratio is set to about 12:1. This means that all signals above -9dB will be compressed at 12:1 and that's certainly not limiting - which is what you would need to do to stop anything going above that level. If you whack the ratio control up full (30:1), then you will have effectively limited the signal at -9dB.

For what you appear to be doing, you might want to experiment with using the Hard Limiter, which will achieve that for you automatically. Same thing applies as far as normalizing first, though. In many ways the Hard Limiter is easier to set up because you only have to set up the peak value of your resultant file, and the input boost.