I understand what you are saying, but nvidia themselves said its similar, not identical, and I tested it, just like you said to do, and it clearly shows a difference down in the pixel levels of the two images.
Ok, here..... The compression ratio of the overall image of DXT5 is 4:1 and DXT1 is 8:1, so YES, the RGB channels are stored the same, but you have to think of the OVERALL compression of the file. 8:1 vs. 4:1, that is where the difference comes into play.. SO if you re-open the images, after saving them, because of the compression difference, you see a different pixelation of the two images.
I'm not aurguing, I think we are basing on the same thought process, but forgetting the main compression of the image.