I think you are both right - the springs aren't 10% stiffer, they are 20% stiffer - I've ridden a bike with a set of the lewisport springs and it was stiff for sure, definitly 20% at least.
I think the confusion must have originally been if they were sold as 10% each for a total of 20% or something. Well that's silly. You mean if I put a 10% stiffer spring on the back I'm up to 30%??