Sacha writes
One of ours fascinates us. It's growing at the base of a mature tree
so must be in competition for water and nutrients, though it's
obviously an old and well-established Wisteria. However, it scrambles
right to the top of the tree every year and flowers like a mad thing.
It gets absolutely no attention whatsoever, not even pruning, because
we can't get at it. So it defies all that is said and prescribed for
Wisterias and somehow gets away with it!

No, it's consistent. Natural habit is to climb till it can't climb any
further, then the shoots bend over (telling the plant there's no
support, therefore it must be at the top of the canopy) and flowering
side shoots are stimulated.
The pruning regime is to stimulate the side shoots and kid the plant
into thinking it's at the tree tops in an ideal position to flower.
--
Kay