The gearing in the two vehicles may play a part and the fact that the cars were loaded with passengers (extra weight).
Toyota
4.235:1.....1st
2.36:1.......2nd
1.517:1.....3rd
1.047:1.....4th
0.756:1.....5th
couldn't find rear final drive ratio

HHR (yay)
2.96:1.......1st
1.62:1.......2nd
1.00:1.......3rd
0.98:1.......4th
final drive ratio...3.63:1

Since a V6 would not have to work as hard under a load, and it's 5th gear is overdriven at a greater rate than the HHR, I could see it getting better mileage in those conditions.
Doesn't the Highlander have a constant velocity differential that adjusts according to speed?

 

You've got your ratios mixed up a little, the 0.98 4th is for the F23 manual transmission, it's .68 for the 4T45. Final drive ratio is 3.48 for the V6 Highlander.