Why does the width of a line change depending on its length?

I am using Fireworks for CS5.5 on a windows 7 64 bit machine.

Set the screen resolution at 72 dpi.

Set the line width at 20 (tip_size?) and draw a line 500 pixels long (hard line).

Then, without changing the settings, draw another line 1000 pixels long.

What does your line width or tip_size read now?

On mine it reads 23 pixels - does anyone know why?

Is there a setting that does not change the line width if the line length changes?

It has caused me lots of problems of late, and I am sure that it has not always been so.

Surely the line width should not change depending on the length of the line drawn.