I wasn't trying to shut you down. It's just easier to get an answer when you guide us towards what you're seeing. I thought there might have been MD on the reverse of the 1991 D, but the T in STATES might just have taken a hit since I don't see the doubling elsewhere. Again, it'd be easier to give an opinion if you direct us to what you see.
That "Sorry!" wasn't sarcastic either. But if you want a bit more of an explanation, have a look at the first picture of the 1992 D cent. There are other plating bubbles visible in the field above the date. This is fairly common in the copper plated zinc cents and adds no premium.