There is no inherent relationship. I could imagine a situation where BA provided e.g. 4 seats for awards in a cabin, and these then get used up even before a single paid ticket was sold. Alternatively it may be for commercial reasons that there are no rewards seats at all for flights which are always busy (e.g. Friday evening flights). On the other hand I've also seen flights were there were only expensive CW fares for sale which still had award availability in CW.