Some of it has to do with the location of T8 as 22R or 31L are frequent departure runways in the afternoon. This typically means AA aircraft taxi on Bravo counterclockwise around almost the entire JFK terminal complex before joining the line of aircraft waiting for the active runway - which sometimes stretches down almost the entire length of Charlie. So, the taxi time would be a bit longer than if one was originating from, say, Terminal 1 or 4.

Additionally, JFK can have some pretty horrendous congestion in the evening which adds considerably to the taxi times. Fortunately, it's gotten much better over the past year ever since they impoved the process to cope with the 13R/31L closure. I used to fly on the evening JFK-DCA flight regularly a few years back and the taxi time almost always exceeded the actual flying time.

-FlyerBeek