It's the amount of employees. An Oil companies has hundreds of employees to pay with nearly the same revenue while wal mart has something like 3000 employees to pay plus maintaining all of their stores.
These are issues oil companies, gas companies, microsoft, apple, etc don't have to deal with to such a large degree.
For all the feminists out there! interesting fact of the day...
Women make only 80% of the salaries their male peers do 1 year after college!
10 years after college, women earn only 69% of what men earn, in spite of the fact that women have slightly higher grade point averages than men in every major, including science and math -- all according to the American Association of University Women Educational Foundation.