Hydrocarbons Parts Per Million

Had the wife's 18-year-old Vauxhall Corsa MOT today and it past the emissions but the HC ppm reading was 100 which was well within the 200 maximum permitted. For comparison I then looked at my Roomie's ppm reading from last October. I found the ppm was 111, even higher than the 58k 18-year-old.  Considering at the MOT the Roomie had done only 24k miles I was surprised that the ppm reading was higher.