Why has my fuel economy dropped on my 04 Yukon SLT?

Prior to my completed 100K tune up 2 weeks ago, the economy indicator was telling me I was averaging 15.4mpg mostly in town driving .Right after the tuneup I took a road trip and it jumped to 17mpg. I was thrilled. I realize highway driving can be higher. Now, and mostly city driving short distances again, I am averaging 13.3mpg. So, why the drop from 15.4 to 13.3 before the tuneup? Before I go back to the garage where I had my major service work done I would like to have some ideas of what may be happening.