I had a feeling that was probably the case

My track record for being offered the WT to WT+ upgrade on the PHL run is actually pretty good (near 100% in recent months, although budgetary considerations meant that I didn't take it every time): they have almsot invariably been offered at booking, but then if not taken up sometimes disappear until a week or two before travel when they may pop up again.

I'm wondering if the POUGs appearing nearer to the date of travel is related to BA's rostering of aircraft: the ZZZ birds are frequent visitors to the City of Brotherly Love and only have 24 seats in W. By some miracle I have always managed to avoid these (my luck has to run out before too much longer). However the VII 772s are also very common here and have 40 W seats. I suppose it would make sense, once BA has a better idea which aircraft is likely to be operating that particular flight, to sell W on the assumption of a ZZZ clunker but then release the additional 16 seats if/when they think it's more likely to be a 40W 772.