Mirror Components! How does SWx define pivot point?

Hi,

In an assembly, I need to mirror components.

Ref: 2012-03-08 pic01.jpg

My question is; what is the pivot point? How does SWx define this point? (In the first window, I’ve given a mirror plane only)

My issue is, as soon as I click OK, mirrored part slightly jumps to a side.

Ref: 2012-03-08 pic02.jpg

SolidworksAssemblies