creating new projects leads to this screen, what the hell?!

So I have some .dds texture files. I noticed that when I open them in GIMP and export them again without modifications, something in the file data changes even though I keep all the settings the same (mipmaps, DXT1, etc.)

The file size is the same, and visually everything looks the same. Yet the file hashes are different, meaning some data was modified. Furthermore, when I try to compress the file even further (e.g. using oodle), the GIMP-exported version always has a slightly larger size.

Anyone know what's the issue here?