If you check the date code on the tire, you may find out the tire is a bit older than you think!
Depending on where you bought the tire, it could have sat in a warehouse or in the shop for a while.
The age code is a 6 charater code on the side of the tire, it will be 3 letters, followed by 3 numbers. The first two numbers are the week, and the last number the year.
A tire manufactured THIS year would have a manufacturers date code something like this ABC225
22 being the 22 week, 5 being 2005.
The 'age' thing is also a factor in car tires, but I think a little accelerated in bike tires, since stickiness outweighs longevity.