I noticed something interesting today. At the start of the departure-side walkway in T5A, there are some monitors above where the moving walkway starts. Those monitors seem to show the same information you would see at the T5B/T5C Flight Connections area (e.g. "Go to B Gates").
At 13:00 or so, the monitors in the main terminal level (in both T5A and T5B) showed my 16:00 flight to Philadelphia as "Gate shown at 14:45". However, the monitors at the start of the walkway said "Go to B Gates". That confirmed what I already suspected, which is why I was on my way to the T5B lounge.