That only charts cities. I strongly suspect that the answer is just that there are unmatched women who lives in more rural/suburban areas.
For people between 18 and 44, the number of men very closely matches the number of women. So, ignoring polyamory, the the proportion of singles across America must be very gender-equal. Since that shows that the proportion of singles in cities in male-skewed, the proportion of singles elsewhere must be female-skewed.