fiesta speedo

Approaching one of those trailers setups that read your speed the other day so I sat the fes on exactly 80 kliks and guess what? It showed my speed as only 75k. So at an indicated 100k its only doing about 94k. No wonder when I set the cruise to 100 kliks all the traffic is passing me. So it looks like i need to set it to 107kliks to be at the 100 speed limit. You would think that in this day and age a new car speedo could be made more accurate than that.
I also checked the capri speedo on the same day and for80k on the speedo it said speed was 79k. How come an old cable speedo can be so accurate and modern electronics so far out?

in the wifes toyota with the GPS the gap appears to increase the faster you go i.e. the speedo is almost spot on at 40 k's but close to 10 k's out at 100 k's

It appears to be closer to the mark when you go through a speed display thing on the side of the road