When you normally see a negative loss figure, it would indicate a "gainer". This is not the case with the SimpliFiber; it is the opposite to what you would normally expect.
A reading of -1,3 dB on the SimpliFiber is a loss of 1,3 dB.
If the SimpliFiber reported a loss of 1,3 dB, this would then be considered a gainer which of course is impossible when testing a passive cabling system. The most likely cause is that your reference setting was not valid and/or you did not allow the source the warm up/stabilize.