and MH370 is the only missing 777 right now, but authorities refuse to say that the piece is from MH370? That doesn't make any sense.

http://www.cnn.com/2015/08/31/world/mh370-investigation/index.html

The flaperon is from a Boeing 777 -- and MH370 was a Boeing 777 -- but the company can't say with certainty that the flaperon found on Reunion Island comes from the missing plane, the source said.

MH370 is the only Boeing 777 that is unaccounted for after getting lost over water in the region.