I'm not an expert here like WB or Kelly, but since he is gone racing in ID and until WB weighs in, I'll take a stab.
Shortening the tube moves the inlet to a different vacuum zone and lessens it's effect on the mid+ throttle opening mixture. The MS metering rod is richer on the larger throttle openings and so it takes more control of the mixture there.
So when the shorter tube is used with the MS needle, the needle is more in control and more of the Lectron metering rod theory is realized (better atomization etc), because the PJ screw isn't open as far.
Lectron power jet metering is like conventional carb MJ metering and doesn't atomize as well, so if your Lectron PJ screw is screwed out (more than a turn), you are missing out on some of what you bought the Lectron for.
.800"= 20.32mm
.400"= 10.16mm