Greenhill, you calibrated correctly to (Dolby) Reference Level.
Remember Dolby Reference Level is 105 dB PEAK. A peak occurs when the digital signal is as high as it can be and that point is known as 0 dBFS (0 dB 'Full Scale'). The test tone you used has an average level of -30 dB or 30 dB below full scale. When you played that tone, you adjusted the channel trims on the receiver until the SPL meter read 75 dB. Thus when playing a signal with an average level of -30 dB, you get 75 dB SPL at the listening position. [Incidentally, the 'standard' (although not universally followed) for dialog level in a movie is -31 dB, which is why I personally prefer to use the receiver internal test tones].
All of this is when the volume display reads 0 dB because that is the point you chose:
When the signal hits a peak (loud music transients, explosions, etc) it will be higher than -30 dB. As the signal gets 'hotter', its digital level gets closer to zero. When it hits 0 dB, you get 75+30=105 dB SPL. Let's say the music of the soundtrack suddenly gets a little louder, but not quite full scale, so the level is now -20 dB - at that point you get 75+10 = 85 dB SPL (the input signal is now 10 dB hotter than the one you used to calibrate). When the soundtrack's average level is around the level of the tone you used, the SPL at your listening position will be around 75 dB.
Play a CD where the average level is MUCH higher than a movie soundtrack (average -10 dB these days) and your SPL will be 125 dB when the music hits 0dB because remember you calibrated with a -30 dB tone and are now playing something that is 20 dB hotter to begin with. That is why you don't have to turn the volume up as much when playing CDs compared to a movie.
One of these days I'm going to take screenshots of the waveform in Sound Forge from various songs on CD so people can better understand all of this junk.
This is why people find digital audio confusing:
0 dB SPL - threshold of hearing; practically silent.
0 dB FS - maximum level of a digital signal.
0 dB relative volume display - arbitrary point near the max range of the volume control, but once calibrated it relates to a specific output SPL given you know the level of the input signal.
0 dB channel level - the point at which the input signal is neither boosted nor attenuated. After calibration, these are rarely zero - they can be either plus or minus - whatever it took to get the output SPL you were looking for on the SPL meter.
0 dB on a fader or mixer -the point at which the input signal is neither boosted nor attenuated.