Lens initial light




I've got a little problem with zoom lens and it's initial light measurment.
What I mean when I say initial ?

1. My test conditions: lens with full opened blades
2. Measure light when lens are connected to body but no twisted for locking (camera displays f=1.2)
3. Measure light when lens are connected to body AND twisted for locking (camera displays f=given on lens)

If I think correctly parameters in my viewfinder should be the same (or slightly different).
In point 2'nd I've got real light, and in the 3'rd should be the same, BUT IS NOT

In point 3'rd I've got about 2/3-stop longer time displayed by camera?
I've tested this on two bodied (Aria, 137MD) with several zooms (Yashica, Tokina, Sigma - const./variable light, but no Zeiss).

I've NEVER had such situation with fixed focal lens and this is worry me - who lies, and why?

AND questions are:
Does it a matter for photos (especialy slides) or this is normal and I shouldn't worry about this?
What about accuracy?