Cartridge BBs - how tight

[PJay]

When I fit bottom brackets I tighted the drive side cartridge fully
(so it's nice an tight) and then the non-driveside cup/adapter so that
it feels similarly tight. I've been fitting BBs this way for quite a
while without any realy problems but today I noticed that when the BB
is fitted the axle turns more stiffly than it does unfitted. I've even
used a torque wrench to ensure that both sides are tightened equally
and in accordance with Shimano specs.

With the cartridge fully installed there's no discernable extra drag
on the axle, it only appears when the cup is fully tightened as well;
it needs to be quite loose for there to be no effect.

It extra drag is only really noticeably when twisting the axle with my
fingers, with the cranks installed the bike rides fine and since I'm
well into my fourth year on the same BB which has been fitted this way
it doesn't seem to be adversely affecting the bearings.

Why should tightening up the non-driveside (which is a plastic cup)
effect things this way? Will tightening the non-driveside cup also
tighten (and thus over tighten if already up to torque) the driveside
or do both sides tighten/loosen independantly?