scale and position related coordinates in GL

Hi all,
i'm very surprised with this tonight.
can you reproduce it?

here's a very simple patch.

a plane (scale 1. 1.) next to another exact same plane.
Left plane's position is 0. 0.
The x-axis offset of the right plane, has to be 2., in order to get them next to eachother,
without overlapping.
Why isn't it simply 1. ?

I'm thinking something's wrong, somewhere, in my settings.
i can't see what ...

Thanks for your clues ... :)