That's quite interesting, my experience is quite the contrary. For me, with the earplugs in my T seems to amplify, as there are no outside noises masking the T, making it seem louder. How it could get more quiet, I don't know.
I experienced that too but not always. The ringing appears less raging with ear plugs in because my ringing seems to be exacerbated by competing sounds in my surroundings at times.
I think that's because with earplugs your brain can not compare T with other sounds, a thing it can do without earplugs. Without earplugs, the T is always here even with normal sounds so there is a comparison between the T and sounds, and because iyou can hear it with normal sounds it seems loud.