Apologies if I'm reading between the lines and have got the wrong end of the stick, but I don't think this (STN selling faster than BHX) should be surprising?
Of the three Primera launch bases, CDG is larger than STN (therefore you would expect CDG to sell quicker than STN) and STN is larger than BHX (therefore you would expect STN to sell quicker than BHX).
On this basis, and in the event Primera suffers delays for whatever reason that require service cuts, you'd expect BHX to feel the brunt first because it's cheaper for Primera to resolve.