The original statement is correct. Most speedos display a speed that is approximately 10% greater than the true speed, in other words 10% +/- several %. Or to put it another way ~10%. That's what about means in this context – it is an adverb to describe a that a quantity has been stated in an approximate manner, i.e. with a degree of imprecision.
A minority of speedos (i.e. those that do not fall within the 'most speedos' group) still display a speed that is greater than the true speed but they fall closer to a being a much smaller % out or a much greater % out (say 0-4% and 16%+ out). A tiny proportion of speedos display a speed lower than the true speed when travelling at normal cruising speeds, which is illegal.
To put it another way, all speedos bar those which are illegal overstate the true speed by approximately 10%. The use of the word ALL does not nullify the use of the word APPROXIMATELY.
P.S. Do pedantic posters that insist on staying on the same course of argument sit really far forward hunched over their keyboard, nose almost touching the screen?