OK, this is a weird one. I take my blank Numbers sheet:

...and export it to Excel:

Whoah, that seems a little large. Let's rename the Excel doc to .zip and have a look inside:

Digging a little deeper:

Well, we just found 99% of the file size. But what is it — some sort of embedded preview image? Let's open it:

Nope, just a 635x635 PNG of random noise. Anyone have any idea what's going on here?