Plotting to PDF, same layer: different line weight error?

Ok, so i have a plot table set up asigning different line weights to different colours. I have layers set up pointing to the specific colours.

Eg Layer Exterior Walls - Colour White

Colour White -> lineweight .25mm

There are some lines on this layer (and on others) which appear different to the rest when plotted, as though they aren't following the line weight rule.

Link to PDF: http://dl.dropbox.com/u/8752503/test.pdf

(Only the exterior walls layer is selected but this error is on multiple layers)

Ian