As explained to me by a London maker's manager, the "matched" part was so as to make two totally identical guns in balance, feel and tactile sense that the user would not be able to know which of the two he was shooting. The idea being that no perceptible difference in the guns would be allowed to ruin concentration during a drive.
To get that kind of identical feel takes skill and time so a gun from a true pair is likely to have had more work lavished on it.
I wonder whether favoring one of a pair and using it more changed its tactile sense in the hands (due to more wear), and thus ruined the matching that was the original goal.
