It looks like AA might not be adding the standard 3rd flight DFW-LGW for the summer. It comes onto the schedule on March 23rd, and as of April 1st its capacity is zeroed out from April 1 through the whole season. This flight is the 3pm 767 departure (flight 80). The other two flights are usually 777s, but the later one, 78 is now showing to be a 767. Flight 50 is still a 777. I'm thinking the new Shanghai startup is what's driving this. It starts in April, which is about when this happens. They need the 777, so they take it off DFW-LGW and downgrade it to 767, like they did RDU-LGW to run the new ORD-DEL and DFW-KIX.
That doesn't fully explain why not 2 767s and 1 777, however, but the fleet might be stretched too thin this summer. Anyone have any insight?