Q: clip volume vs master volume

Rely on the meters, not the waveforms.  Waveforms are a general representation, not that accurate.  The level of a single clips is that clip's specific level.  But, when you have other clips playing at the same time, even if they are all limited to the same peak decibel level, the overall volume will rise.  Audio is cumulative, as in, one source at -6db is -6db.  Two clips at -6db will be up around -3db(-ish) or in that neighborhood.  So if you have two or more audio sources playing at once, both need to be pulled down a tad below what you want the overall resulting peak db to be.  Also, for some reason, I find the Limiters inside FCPX to act oddly, as opposed to what happens inside Logic Pro and other audio apps that use Audio Unit plugins.

"Rely on the meters, not the waveforms.  Waveforms are a general representation, not that accurate."