I think you are assuming slowly merging onto an empty hiway with a long on-ramp and then maintaining a speed. Around town the best technique is to use your engine efficiently (if the transmission allows it) and avoiding the brakes (don't over-accelerate, coast to stops). You also have options if the hiway is mostly empty, such as drive slower and pulse and glide, drive with load, etc.
Also there appears to be some issues with the accuracy of the imid
What is your Civic SI's best MPG? | 9th Generation Honda Civic Forum
It may not be recording fuel consumption directly, unfortunately (indeed since you are inferring fuel used from displayed mpg...)
Experienced hypermilers understand the compromises involved. I've p&gd a beater 5 speed all over the state to the tune of 55mpg. As well as route selection becomes a factor also, i.e. efficiently pulse and glide on surface streets, or a 65mph burn on the hiway, might only make a 4 minute difference in commute and cut your mileage in half.
It's complicated... My prius (arguably a cvt) loses a lot of efficiency in the transmission if I go over 2400 rpm at lower speeds, but my 5 speed civic doesnt.