Hey Ming Rider, the problem I see with judging the contrast difference you see is that it is a fully adjustable characteristic, it can be matched across all 4 development regimes by adjusting say the time a bit.
The difference is effectively irrelevant, the adjustment to fix that contrast difference is simply part the normal refinements we all do. It is not an inherent difference.
But a controlled experiment is essential to find the required adjustment?
"All I ask for is an M5 with a fast lens, a roll of HP5 and a street to shoot her by."