Multiple Correct Solutions to Puzzles Not Accepted?

On this puzzle http://en.lichess.org/training/25523 for my third move I played Rb8 and was told I failed the puzzle.

However Rb8 here is approximately -8.5 and has the point of the problem in mind which is to win the white bishop on c1.

The correct solution given is Qxc3 (which I intended to play on the next move) which is also approximately -8.5 and perhaps just a few centipawns superior a move.

I don't understand why sometimes the trainer tells me "good move but you can do better" but on so many problems only accepts one solution when more are available.

I know that often problems are set such that I could play a move that is +5 in my favor but if I'm missing a mate in 3 then I get puzzle failed. I could quibble with that but I at least like that it teaches me to always look for something better.

But sometimes, such as with this puzzle, I just don't understand the difference between moves accepted as winning and moves given as failing.

I'm wondering now if it has anything to do with the computer needing to follow a forced line in the puzzle trainer in order to continue the problem.

For instance when I play 3. Rb8 the computer could play several pawn moves that are all similarly losing to handle their bank rank issue while 3. Qxc3 indicates one clear reply.