A couple of suggestions, could it be, Dunlop was experimenting with a different rubber compound on the tread about that time? Tire manufacturers do this on occasion, as they can track the tire via the serial at the end of its life cycle. Or maybe the regular builder was away for extented period of time(due to illness, vacation leave etc...)and some one new took his/her place and used the wrong spool of rubber for the tread pattern in the building process. Every tire have a number of different compounds for different areas of the tire. The only way to find out is to complain to Dunlop, I doubt they will come right and admit they had a problem on a certain run of tires, at the most they may offer you a prorated adjustment on a new set. If they do,(the offer won't be very much) you may decide to turn the offer down, as others here have given you good advice for a alternative make of tire.