In the page model, a heading says it needs only one line of vertical space, so if there’s a tiny bit of space at the bottom of the page it’ll get orphaned. (Vertical box space shown as ! and % for the heading and paragraph, respectively.)

  Page 1         Page 2
  ..        Paragraph..%
  ..        ..text.    %
  ..                 ..
  ..                 ..
 !Heading

When instead it should be moved to the top of the next page:

  Page 1         Page 2
  ..        Heading    !
  ..        Paragraph.. %
  ..        ..text.     %
  ..                 ..
                     ..

Rather than being honest about needing one line…

  Heading     !
  Paragraph..  %
  ..text.      %

…the heading could instead claim it needs three lines, which would ensure it would never be orphaned:

  Heading     !
              !
              !
  Paragraph..  %
  ..text.      %

But now you have a big gap below the heading.

If you could then shift the paragraph up from where it should be in the flow such that the vertical space of the heading and paragraph overlapped…

  Heading      !
  Paragraph..  !%
  ..text.      !%

…then you’d get a heading that would never be orphaned on one line, but which looked as it if only used one line.