Tyre load rating

Can any one enlighten me as to why if I run a 235/70/16 tyre the tyre load rating must be equal to or greater than 106, however if I use 255/70/16 the rating must be equal to or greater than 111.
I would of thought that regardless of what size tyre is used the load rating requirement would have been the same.
This was only brought to my attention when I called an independant tyre retailer to get a comparison quote and he 'said he wouldn't sell me the tyre I wanted because its load rating (which was 109) was lower than the tyres on it (111). The size I am looking at is 255/70/16.

I ended up getting the same load rating as the continental factory tyres, I had to reduce the speed rating to 180kmph as continental were the only tyres in Aust that had that load and speed rating.

I was told the wrong rating will affect your insurance to be careful, but the insurance broker i have said to ensure you have tyres rated at a load rating for at least what you are carrying and the same with the speed rating. Apart from that there is no problems with insurance.