Speedo only gives an indicated speed, you may both have the same top speed as each other. Suspect that the margin error on a speedo at that sort of speed is almost identical to the difference you're quoting between the two vehicles. Many other factors would also prevent the two vehicles having identical top speeds, gearbox ratios (someone may have fitted a different box at some point to one of the vehicles), diff ratios, drag coefficient (due to different body configurations), rolling resistance and health of the engine (loss of compression, coked up, worn tappets), binding brakes. Life's too short, I wouldn't worry about it, if you enjoy driving ya landy then job's a good 'un.