I recently went on a driving trip from near Houston, TX to Kansas City, MO. After I got back, I looked at the mileage I got. I was surprised to notice that I got noticeably better mileage with the gas I bought in Texas. My car takes premium gasoline. Is it possible that Missouri blends in ethanol and Texas does not? Any other ideas?
It’s entirely possible that the one pump you used shut off faster than the other. You could also have had a head wind in one direction and a tail wind in the other.
Unless you did a rigorous multi-tank, multi-trip comparison, tanking at the exact same stations, same pump and stopping at the first click, any difference in gasoline will be completely nullified by the sloppy measurement.
We get posts like this frequently and cannot possibly comment one way or the other.
In any case, the gas quality is much better controlled than your mileage measurement methods.
More up hill one way and more down hill the other is another factor you may not even notice. ( At my 6500 ft. I get better mileage leaving home to go ANYWHERE and worse mileage climbing back up. )
Traffic differences, like the drafting effect, is not obvious to the average driver either.
It is possible one pump was not as accurate as the other. It is also likely that you got better fuel economy in hotter weather. It could also be that southeast Texas has less hilly terrain than Missouri. There are many possible factors that could affect fuel economy without your knowledge.
“It could also be that southeast Texas has less hilly terrain than Missouri”
Southeast Texas has less hilly terrain than just about anywhere in the world except maybe the Bonneville Salt Flats. 
Missouri has mandated 10% ethanol in all fuels purchased in Missouri for several years now. Gov Matt Blunt initiated the program, and the legislature bought into it, when he was in office.
That may explain the difference. Kansas, Oklahoma, Texas don’t have this mandate.
Perhaps you were driving with/against the wind? HOU is slightly west of St.L, and weather comes up from the Gulf frequently.
The direct route goes up through Oklahoma and Kansas. One could go up through Missouri via Joplin but western Mo. is prairie and there are no hills to speak of.