Actually the population can't be used as total voters as not all are registered or elgible. The number of registered voters in Houston is just under 1 million of which about 16.5% of voted. Any candidate with an unified effort and 10% of the registered voters could have won going away.
Mayor In: 100% Gene Locke 73,331 47.2% Annise Parker 81,971 52.8% Total 155,302