I have the same situation with my car! all 4 years are 5.0 liter PGMFI engines, with AOD transmissions. No mechanical changes, but check out my EPA's Highway mileage changes between 27, and 24!! That's like 11%
The EPA periodically revises the way they calculate the gas mileage ratings. You can use them to compare cars of the same year, but not cars across different years. The way they calculate is very arbitrary and unscientific:
Look at the last paragraph. That's where they admit that they "adjust" the ratings downward to more closely match the real world. I read somewhere the adjustment is on the order of 20%. They also make up new rules sometimes like the air conditioner must be on, if equipped.