that's their secret: it all sells. some better than others, but once sets start going on sale (moreso clearance), that shit disappears. beyond the standard 20% off, if the prices are 40-50% off in stores, it's all sold pretty damn quickly, ime.
there aren't a lot of flops, afaik. the recent Black Panther bust was a bust, and the foosball set was more of a boosball. I really liked the Queer Eye set, but I think Lego misjudged the level of interest it would have. I'm happy I picked up a few of those, though; great set for parts!
I've also heard that Walmarts will sell off palettes of sets to employees for really cheap. either way, it all sells, and often eventually find its way back into the Lego ecosystem, ie bricklink.