That could be a function of the age of your car vs the age of theirs. Cars made post-2008 are supposed to have speedos that inflate your speed by 10%, something to do with convincing people to drive more slowly. You can check it by driving past one of those passive aggressive speed signs you see outside villages that tell you your speed. Your speedo will say one thing, the sign will say another.
If, like me, you drive a pre-2008 car (saying nothing about my bikes *cough*), you'll be doing the speed your speedo says you are, but if you're behind someone who's sticking to what their post-2008 speedo is telling them, then yes, they will only be doing 27 or so. If they don't know their speedo is lying to them, they will go slower than they think they are.