Greater Than The Sum is just a cropped version of one of the SOTL calendar images. And I guess the guy who did the calendar image didn't get paid for the re-use, so it's lazy and cheap.
Which is why finding out is better than guessing. If you check the copyright page for GTTS, toward the bottom it says "Cover art by Mojo" (the pen name of digital artist Adam Lebowitz). Since he was credited, that means he was indeed paid for the reuse.
And it wasn't the result of laziness. There were some behind-the-scenes issues that delayed the cover art process, and ultimately they had to reuse an existing piece. But it's one of my favorite SOTL images, and I feel it fits the tone I wanted the book to have even though it doesn't match any actual scene within the book, so I'm satisfied with it.