The mousover always shows a contiguous section of the underlying file. So you're seeing a constant-length section of the space-filling curve centered on the cursor as you move the mouse about. The hex to the right is that same contiguous piece of the file. Sort of neat, actually.

I think maybe they're referring to the logo on the main page, not on a binary file? But that's an answer to another question I definitely had.

Oh, right! That's just a bit of fun - a random pattern (picked to be vaguely visually pleasing), which is laid out on the Hilbert curve, and then offset by a calculated amount depending on the mouseover. So I'm just shifting the same pattern forwards and backwards in a ring buffer, basically.