It's purely based on quantity not quality and thus more a comment on the gastronomic markets and the geography of these cities than the quality of the food offerings.
What you can take from the numbers is that Washington D.C. has a *lot* of food places per capita - which isn't surprising given it's a city of work and play - rather than a city of residence - for large quantities of young, unsettled people with good incomes and active social lives. Meanwhile Philadelphia doesn't have a lot of them per capita which perhaps indicates that Philadelphia doesn't have as many people of above type as cities like D.C. and very importantly that it encompasses large swaths of primarily residential space inside its city limits.
If you look at the statistics, the city of Philadelphia encompasses around 25% of the population of its metropolitan area. In comparison, the city of Washington only encompasses about 11% of its metropolitan area and the city of Detroit, which ranked very highly in the burgers and pizzas categories, encompasses only about 16-18% of its metropolitan area. In other words, Washington and Detroit host - in relative terms - more eateries for people who don't actually live in the city itself but work and socialize there (due to entertainment venues, stadiums etc.) than Philadelphia.