The RIAA EQ specification reduces the bass and boosts the treble, which as you say is reversed by the phono preamp. I was going to say it's not an exact science and therefore it's open to errors, but that would be strictly incorrect because it really is an exact science, feel free to look up the RIAA EQ specification on the net.
What I really mean when I say "it's not an exact science" is there are a million and one potential variables that could affect the 'decoding' / re-equalisation, so it's probably true that it doesn't happen as seemlessly/perfectly in every tt->cart->arm->-cables->amp combination as it will in others. I'll give you that one.