When the fiber after a splice scatters more light back than the fiber before it, usually because of a smaller mode field diameter, the OTDR sees more backscatter after the splice and reports negative loss. Shot from the other end, the same splice reads a larger loss than it really has. Averaging the two directions gives the true splice loss, which is never a real gain.
Splice 14 is a gainer from A, 0.12 high from Z, average it out and it's a 0.03 splice, leave it alone.
Rookies resplice gainers trying to make the minus sign go away. The fix is a second shot from the other end, not another splice.
Related terms
Bidirectional averaging
Testing a fiber with an OTDR from both ends and averaging the results to get true splice losses.
Mode field diameter
The effective width of the light spot traveling in a singlemode fiber, slightly larger than the physical core.
Backscatter
The small fraction of light scattered in the fiber that travels back toward the source, which OTDRs measure.
OTDR
Optical Time Domain Reflectometer: a tester that maps a fiber's loss and events by timing light reflected back along it.