Fair play to Charles Stross for just wanting to write.
I had a tech book out in the mid-90s from Addison-Wesley, and I got quite a way into negotiating with O'Reilly in the early noughties. The tech publishing sector is largely driven by techies for whom a publication history is of value as a check mark on their resume, rather than people who write for a living: consequently there's a race to the margins to document extremely esoteric niche products, and this triggers a race to the bottom on advances/royalties (it's hard to justify paying up a big advance for a book with a market of maybe 2000 readers).
Even a badly paid midlist SF novel can expect about twice the advance O'Reilly offer for a tech book: the potential audience is much bigger, and the incentives for publishing in the first place are different.
Then I saw your user name. I suspect you probably know a thing or two about this topic.
But I have to admit that the direct impact of my tech books has been a boost to my career, not my bank account.
I have no idea how A-W treated you as an author, but as a reviewer on that book, I was very well compensated by them, so I assumed they treated authors OK as well.