I cannot be sure but i am guessing as a result of them being "10% bigger than the rest" or something like that, or more precisely they were a standard weight once, but then a percentage would have been added "free".
You may be interested to know that the size of the multipack bags has been getting slowly smaller over the last few years, (currently 25g, used to be around 28g) as Walkers etc. decided to have a price war but obviously needed to continue to make a profit, so sneakily shrank the bag contents (note the bags are still the same size!)