Let's assume a scene with two people talking. One person very loud in the foreground, the other softer in the background. Now we play that scene 10dB below reference. A loudness based volume control now boosts lows and highs to restore spectral balance. Now the person in the foreground stops talking. Does the person in the background now need additional or different loudness compensation?
The dynamic aspect of Dynamic EQ takes care of it by mapping the softer (or louder) parts in the input signal onto the right psychoacoustic "curve" since Dynamic EQ looks at the Volyme dial AND the level of the (content) input.
A static approach like THX Loudness Plus would apply the same bass boost curve independant of if it were a soft or loud part in the input signal since it only looks at the Volume dial.
You can test it yourself by running a test sweep in REW or similar and using the same master volume setting but varying the input signal level, such as -10dbFS, -20dbFS and -30dbFS for Dynamic EQ and THX Loudness Plus.
You will see that the curve is shifted 10dB each time with Dynamic EQ and the slope is changed, ie different psychoacoustic curve based on input and volume setting.
But with THX Loudness Plus, it will be the same static curve, just shifted 10db each time.