Trouble with display of spot color

[local1]

I have a two color spot pdf. Color changes rather dramatically when overprint preview is toggled. All items are set to knockout and do not overlap.

For example create a blank PDF in Acrobat.
Draw a large rectangle - color it 100% of Cool Gray 6
Draw another large rectangle - color it 60% of 5463

Now with these two objects side by side toggle between overprint on/off.
What I see with overprint on is correct but I do not understand why in this case it makes on bit of difference. With overprint preview off the 5463 is way green.

I've tried Acrobat 7 & 8 on OS X

Can anyone tell me what they see or what I may have setup wrong?
Thanks

[Joe]

It does it too when you open output preview...which automatically turns on overprint preview I think. Acrobat 6 did it too. Not sure why.

[local1]

Well it is good to know it happens to others. It wouldn't be a problem for me if I could force files to be displayed with overprint preview on by default.

Thanks

[Joe]

You can. Turn it on under the advanced menu and it stays on.

[local1]

That doesn't control the settings of the person on the other end who opens it up. Then they call and say the proof file is all wrong. Go thru the drill of checking this setting with them and they go ohh. Try translating this thru layers of sales people and csr's