Having a selection of broken tach and speedo cables in my workshop to choose from, I can say with authourity that the inner lengths are all over the place. I've had one which lasted a half hour before spilling it's guts on the road and another where the inner doesn't come out far enough to actually drive my speedo. I was so pissed off a couple of weeks ago when my tacho AND speedo cables died on the same day that I'm now getting thr gear together to make my own super quality cables. Coated braided stainless outers with PTFE internal coating with stainless fittings. They will look a million dollars and should last forever. Don't know if they'll be a commercial proposition but, if not, the cables on my bike will be better and prettier than yours! The other problem with variable length inners is that if the inner protrudes too much it will damage the instrument. Terrific.
Just another example of the crap quality parts which are foisted on we Norton owners these days.