When worn, a piece of armor in an armor set is constantly checking and rechecking to see if the other pieces of the set are present. When all pieces are present, the stats of each piece of gear need to update (change) their stats to be those of the whole armor set. Even after the whole armor set is worn, each piece is still checking that the other pieces are preset. This is because if a piece is taken off, then the armor set stats need to change again to be those of an incomplete set.
In the case of the Plate of Honor Gorget, when the whole set is present, the gorget gives +3 self repair. When it is worn but not with the other 5 pieces, it gives +0 self repair. In this case, the player applied a +5 self repair to the gorget, then put on the gorget. The gorget saw that it wasn't part of a complete set, and set its stats back to those it would have had when worn without the other 5 pieces of the set, which was +0 Self repair, effectively loosing the +5 self repair that had been applied.
What this ultimately means is that for stats that change on an armor set (in this case self repair) you cannot modify any of the values that the set has, as the system will reset those values depending on the presence or lack thereof of the other set pieces.
Ghost