Different strokes for different folks

I have one hotter romance title that has sold a big five copies on Amazon this month. But when I totalled it up on B&N, I saw it had sold 45 copies. Still not a huge seller, obviously, but I see steady sales of all my hot stuff on B&N. When you have enough titles, even smallish daily sales begin to add up.

Conversely, one of my hotter romances has somehow gotten rolling on Amazon, and has sold over 300 copies there this month. It hasn't sold nearly as well on B&N. I don't know how to explain what does and doesn't sell, but clearly having a lot of titles helps, as does having them available on different platforms.