I have a website which consistently displays differently when opened in two windows of the same browser. With “differently” I mean that one version of the website uses different colours. Here’s a screenshot with a vertical cut between the two versions:

Now, I know about gamma correction of PNGs but this difference affects not only embedded images but also the text colour. Furthermore, this is the same website, displayed in the same version of the same browser (Google Chrome), only in two different windows. Putting the tabs into the same window makes the differences vanish. Pulling the tab away makes the differences reappear.
I’m seriously confused by the difference in display – the data of the websites is bit for bit identical. The address is identical. There should be absolutely no difference in memory between the two instances (except for running JavaScript, none of which is supposed to affect the display of any of those components). Yet they display differently. Closing the browser and re-opening doesn’t change this.
What could cause these differences?
I still suspect that it’s got something to do with gamma correction which Chrome maybe does internally. What I don’t understand is why it does it differently for different windows.