"These T-shirts are very pretty."
Translation:Estas camisetas son muy bonitas.
This happens a lot on Duolingo: 1. I gave the wrong answer: Estas camisetas son muy buena. 2. Duolingo said I used the wrong word, and give me her solution: Estas camisetas son muy bellas. 3. When I go to this discussion section, another meaning is given by Duolingo, in this case: Estas camisetas son muy bonitas.
And the last solution seems the right one to me. So why gave DL me firtst another (wrong) solution to my wrong answer?
You are right, but the solution Duo gave you is right-ish. Bellas and bonitas are similar, I guess. The problem is the way Duo picks the answer it presents. It compares your solution with its set of solutions, and when it finds a mistake, it picks the first of the correct ones (I think that's how it works) and presumably bellas comes before bonitas.
The mods have been asking for a change in this for a while, but it hasn't happened yet.