RGB Colour change after save for web.

I recently had a folder full of images that I cut out and placed on a grey #e9e9e9 background, after using adobe photoshops Save for web feature, I saved them as 356 colour gif files. However when Ive uploaded them to the web I notice the grey #e9e9e9 was ever so slighty different. when I reopened the images in .gif files in photoshop, the grey background was no longer #e9e9e9 but #eaeaea. Im slighty confused as to why this has happened, and is there anyway of correcting this?

It depends on the mode:
Perceptual
Selective
Adaptive
In my tests Selective preserves larger gray
areas e9e9e9 (233 233 233). I think there is no
guarantee, except for web-safe colors (which are
otherwise quite useless nowadays).
Please read the help text
Optimization options for GIF and PNG8 formats.

Best regards --Gernot Hoffmann

...and the source should be already in sRGB.

G.H.