So,in DIBs,the bottom row of the image is the first row of the file,
and the top row of the image is the last row in the file. This is
called a bottom-up organization. Because this organization is
counterintuitive,you may ask why it’s done this way.

Well,it all goes back to the OS/2 Presentation Manager. Someone at
IBM decided that all coordinate systems in PM—including those for
windows,graphics,and bitmaps—should be consistent. This provoked a
debate: Most people,including programmers who have worked with
full-screen text programming or windowing environments,think in terms
of vertical coordinates that increase going down the screen. However,
hardcore computer graphics programmers approach the video display from
a perspective that originates in the mathematics of analytic geometry.
This involves a rectangular (or Cartesian) coordinate system where
increasing vertical coordinates go up in space.

In short,the mathematicians won. Everything in PM was saddled with a
bottom-left origin,including window coordinates. And that’s how DIBs
came to be this way.

资料来源：Charles Petzold,Windows编程第5版,第15章.