Why does Numbers hide a huge PNG file in exported Excel sheets? 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?
I'm not sure why it's included in the export, but this is one of the default "Image Fills":