There's been some debate on this in the past. A couple of points:
1) You seem to regard "runway heading" as unambiguously meaning the heading whose digits correspond to the runway QFU. I'm not sure it's that simple. Runways don't have a heading, they have a direction or track, and it seems perfectly reasonable to interpret the phrase "runway heading" as being "the heading of my aircraft that permits it to track down the runway".
2) When this was discussed last time, I think it was agreed that if ATC wished to rely on the aircraft taking up a heading that was different from the extended centreline track ("straight ahead"), the only sensible thing to do would be to assign that heading explicitly. Thus in practice, the difference should never be an operational issue.