Flattening image changes values of adjustment layers

I'm working with CS3 on a Mac OS 10.4.9, 4.5 gb ram. My image consists of 6 adjustment layers and a background layer. 5 of the adjustment layers (all curves) are set to luminosity and 1 color balance layer is set to normal blending mode. When I flatten the image the tonality in part of the image changes. I also tried merging down the adjustment layers one by one into the background layer but the final result is the same. Any suggestions?

Do you see this change when viewed at 100%? (View>Actual Pixels)

Are you looking at the image at 100% of size (actual Pixels)?

It make a difference to what you see on the monitor when you flatten Layers.

That was it - viewing the image at 25% or smaller exhibits the problem. Thanks