Highway Downshifting Gears.

What is your question?

I drive a few city streets and then jump onto the highway when going to work every morning. When I finally hit the highway, it will switch from the highest gear to the next lowest gear, making my engine rev higher than it should. After driving a few miles, it will then resort back to the highest and most efficient gear. I'm wondering why it is normal at first, then jumpsdown a gear, clearly after the engine is already warmed up, and after it has used the highest gear on the highway a few minutes before.