Differences in plugins vu metering?

[slightly]

I never worry about this before now but
I'm having a slight "hiatus" so while I've free time..

How come many plugins which read in vu metering
Have hugely different readings fed with same
signal levels?

Examples I can think of:
Softtube tube tech compresser with a standard
Drum sample trimed signal..

This reads vu and typical with quite heavy trimming
In signal flickers below "0"
Switch to something different, same signal
But waves Kramer pie compresser needles
Are maxed out....

And to lead to a different subject....

Nebula, I'm demoing lite,
Using Boeing 747 compresser and I thought
Default metering is vu,
nowhere near hot, about -16 with the
Same level trimmed signal being used

which is why also with this program which I'm
Using to demo...
There's hardly ever ANY compression even
With high ration up to 1:9 and really low thresholds
-16 as an example mid fast attacks and pretty slow
Releases.....
Is it also the case with this plugin that's not very
Sensitive like others mentioned u have
To incorporate huge input boosts?

And what's typical the sequence:
set ur compresser and then input boost(nebula)
Or input boost and set the compresser then??

Thanks..

[narcoman]

Its to do with reference calibration. One has to know where 0dBVU is on the dBFS scale.