For some reason tubes seem to be alive in the sense that the sound I hear (whether playing soft or hard, loud or quiet) is similar to that which I get from a dedicated compressor, only one that could adjust itself with every stroke of the strings (if one could do that). The problem with a compressor to simulate a tube amp is that when playing quietly or softly I'd have to crank the compressor and then when playing louder or harder I'd then have to dial it back. An all tube amp just seems (to me) that it does that for me on the fly.
So it appears to me to compress, yet does so dynamically, as if I was fiddling with a compressor on the fly while playing. DVC...Dynamic Variable Compression if you will. NOTE: That's not a real term, I just made it up but that is what a tube amp seems to do for me, and that's what the OP was asking.