Track level is peaking but master is not?

The explanation of exactly why Audition doesn't overload is quite complicated, and many people regard it as akin to black magic, even though it isn't really. All you need to understand is that at any point during the processing in Multitrack view, an apparent overload can be corrected down the line. In fact you can have the master output looking like a solid block of green when you mix it to Waveform view, but normalizing the signal will have it right back to where it should be. The reason for sticking with a nominal 0dB though is because although the software can compensate, your hardware can't - so the signal coming out of your sound device can most definitely overload. This is because it's an integer-based device rather than a Floating Point one.

There are now one or two portable recorders (Zoom, Tascam) that use the same system on their inputs, and they imply that you never need to worry about setting the input gain on them, as it can be corrected afterwards in software like Audition, which uses Floating Point processing. Whilst this is largely correct, it isn't entirely (says he from first hand experience), because they still have to interface with real-world devices called microphones, and you still have to be able to monitor the results without overload - the headphone outputs are equally susceptible.