The left border of the container should be at 30px from the left side of the window: 10px body margin + 5px body padding + 15px container margin.
In IE 5.5-7/Win if the window width is such that all the text is on the right of the float, then the container "eats" the body padding body and its own margin (it is only at 10px from the window left side.) In quirks mode even the body margin is eaten.
In the second case the container has hasLayout, no problem.
In IE5.0/Win this problem doesn't occur, but there is an indentation on the first line of the container, fixed in the third case with display:inline on the float.