I am starting to think it does not matter how much you pay for a battery it seems a bit of a lottery. Only on Friday I had to replace a battery on a friends car that was less than 6 months old, this was a Bosch with a 5 year warranty.
My guess the manufacturers are cutting costs so if they do fail the odd one that does go back is easily paid for by the extra profit.
A lot of people replace there cars every couple of years so unless the paperwork is handed over to the new owner the probably get away with it in a lot of cases, its only the long term and classic car owners who are claiming most of the time.