Then it could really just be a function of having a variety of softer and louder songs. For example, SC has a long 10+ minute song that is not loud (Repentance), a long relatively quiet portion at the start of ITPOE 2, and the song portion of TMOLS is also relatively soft.
The reason that FII appears to have a lot of dynamic range in this graph maybe because it has lots of ballads (Anna Lee, Hollow Years, Take Away My Pain) but a lot of loud metal songs as well (Peruvian Skies, New Millennium, Burning My Soul).
Also, as the y axis appears to be absolute values instead of relative values, the curves are not really comparable. Although the shape could indeed show the distribution. But then again, the shape of the distribution could just be indicative of the turn towards more metal songs from early to current DT, not really of the effect of more compression in the mixes.