It could very well be that lower-income, older people (who would disproportionately live in the southwest) are disproportionately likely to make mistakes when filling out surveys.
Use of a letter-grade system also introduces error, since the value of a particular letter grade is subjective and has varied over time. It could very well be that older people, who received their own grades before the grade inflation of the last generation, have stricter standards when it comes to assigning letter grades to things.
Also, I really wanted the grade inflation hypothesis to be true, but I just took another look at the data and age turns out not to be consistently indicative of lower or higher grades for city services.
Thanks for the comment. Both really valid questions/comments.