The "0db" doesn't actually refer to the receiver readout at all. Sfogg has given the most accurate explanation so far.
Digital audio is stored such that "0db" is the loudest possible sound. All other recorded sounds are stored RELATIVE to the zero db mark. So, -20 db is 20db quieter than the audio format is capable of storing. Most test tones are recorded at either -20 or -30db, meaning that you should calibrate w/ your SPL meter at 85db or 75 db, respectively, to make it so that audio recorded at the 0db mark will play at 105db, and that the quieter sounds will scale down from there. 75db and 85db are NOT reference levels. They're just common levels to match at because they won't piss off your neighbors or kill your ears (mainly the latter, since mixing studios and movie theaters don't have to worry about neighbors, and they're the ones who are defining all these numbers for us to worry about).
The "0" on the receiver often is the manufacture's attempt to line up their readout with all these ideas, and they're often close (for me, -3 or -2 on the receiver is reference, because at that volume, a 0db recorded sound will play at 125db at the listening location... but my speakers are sensitive, so for some people that "0" on the dial may actually correspond perfectly). Either way, the number on the receiver is a nice gesture, but arbitrary.
In my opinion it is incorrect to balance your system based on these reference levels if you don't listen at them (which few people do). It's important to MIX at these high volumes because you don't want to let a bad sound go unnoticed that might end up being heard in a theater, but if you find yourself listening to music and movies with the dial around a certain spot, balance your speakers there, because that's where you want it to sound best. See what your front speakers play the test tones at, and match the other speakers to that. Don't feel inadequate because your SPL meter is reading much lower than 75db. Remember, those test tones are not recorded at full volume!
As for some sources being "hotter," this is true, although it's more of an analogue term, which is a whole other conversation. Not all digital recording will hit 0db. Some won't even come close. It really depends how it was mixed. Dialog can be normalized to a variety of different levels for movies, and if (God forbid) the movie finish without anything exploding, it may be really far from it. Songs can be compressed to push more of everything closer to the 0db mark, which make them sound louder while sacrificing dynamics, which is common practice in the pop world. In their favor, this does maximize S/N ratio as well.
Just some ideas to chew on.