When digital recording was introduced in the BBC World Service (and probably the same elsewhere in the BBC) the peak level - 6 on the PPM - was set to 10dB below 100% modulation to keep well clear of the clipping which would occur if we reached it. I suspect that some organizations are recording much closer to the peak, probably using a limiter to avoid clipping. Also some aound mixers add a good deal of compression which as you will know makes recordings sound much louder even if the maximum peak is the same.
I've noticed with classical CDs that older ones are recorded at a much lower level than many more recent ones: it used to be that considerable headroom was left, but I've been surprised at how far I have to turn the volume down for some more recent recordings. Probably the same fashion applies in pop music - where there was always a tendency to use heavy compression by some mixers. I suspect this is what you are hearing, and it's difficult to deal with. In an ideal world you'd be able to 'normalize' various recordings - not just applying a limiter and allowing it to hold back, and thus compress, louder recordings, but checking each recording for its maximum peak and setting that to a standardised level: I don't know to what extent Sound Check can achieve this but it's probably your only option.