GIMP :: Changes Colour When Switching To Index Mode

I have been using indexed mode to save 8-bit png files for the web, just as one would through the Save For Web interface in photoshop. sometimes it works brilliantly, usually when very few colors. but sometimes it changes some of the most dominant colors in the image. for example, if i make an image with background in #f3f3f3, two antialiased type layers in different colors, flatten the image and switch to indexed mode the background will change to something like f3f201 or whatever depending on the colors of the type layers (even if i choose 256 colors...).

EDIT: ok those exact instructions didn't actually replicate the problem this time that i tried it, but nonetheless the problem exists. if no one recognises it i will save the file it occurs to next time so you can all replicate the problem.

EDIT 2: happened immediately. i have attached a png of the file, even if you set it to 256 colors, switching to indexed mode will change the white background to #fefffc. once again, one could go into the color map and change it back to pure white, but to be completely correct one needs to change every color in the shadows (and antialiasing) that fade into the white, which will also have been offset by the switch to indexed mode.