A study by OZA’s old friend Robin Coulter at the University of Connecticut suggests why. In an article (summarized here) in the August 2010 Journal of Consumer Research, Robin suggests that certain vowel and consonant sounds are associated with perceptions of size and magnitude.
So the whispery “ssss” sound of $7.66 makes it sound like a smaller price than the big “tw” and “oo” and “en” sounds of $7.22.
And this does seem to be contingent upon language. So in a language other than English, $7.22 might be perceived accurately as a better price than $7.66.
In other words, the imagined shape of our mouths and position of our tongues might be just as influential as our “rational mind” when we compare prices.