fill-paragraph problem

[guivho]

When I execute fill-paragraph I get a different result depending upon
whether I've selected the paragraph or not. In the example following, I
use a part of the fill-paragraph description text:

[quote]
Fill each of the paragraphs in the region.
A prefix arg means justify as well.
Ordinarily the variable `fill-column' controls the width.
[unquote]

For the example, I flow these three lines as a paragraph with one
single line:

[quote]
Fill each of the paragraphs in the region. A prefix arg means justify
as well. Ordinarily the variable `fill-column' controls the width.
[unquote]

With the cursor inside that line, (without selecting it!), hitting M-x
fill-paragraph produces following result:

[quote]
Fill each of the paragraphs in the region. A prefix arg means justify as
        well. Ordinarily the variable `fill-column' controls the width.
[unquote]

However, when selecting this paragraph as region, fill-paragraph
produces the expected result:

[quote]
Fill each of the paragraphs in the region. A prefix arg means justify as
well. Ordinarily the variable `fill-column' controls the width.
[unquote]

I am totally puzzled about the white space that gets inserted on all but
the first line if the paragraph has not been selected.

Variables `fill-paragraph-function' and `fill-prefix' are both nil.

The leading white-space gets inserted whether transient-mark-mode is
enabled or disabled.

Is there any way to avoid this behaviour? It's kind of annoying to have
to select the current paragraph before being able to flow it properly.

TIA,

Guivho.