Do people think that US is flying to Europe from PHL throughout the whole day? It's really just a couple of DCA-PHL flights that are funneling traffic to Europe via PHL (unless you want a long layover @ PHL). Moreover, outside of the summer season when flights to VCE/LIS/ATH/SNN aren't operating, most DCA-Europe passengers can use JFK as their connecting point, in addition to PHL.