Open a writer document and add some text in multiple paragraphs.  Select them,
apply style default paragraph from Styles pane.  Now select some of the
paragraphs and apply numbering.  Now we have a context modified style.  Note
that the user has not modified the 'default' style in the styles dialog.  Again
select one of these numbered lists and apply default style from the stylist. 
This doesn't reset the numbering. [even ctrl + shift + space doesn't work here]

As the styles are classified into paragraph/character/numbering styles, it could
be supported that picking the default style from the paragraph section should
not remove the numbering.  But generally a numbering is a modification to the
paragraph also; it changes the indent by default.  

So IMHO, 'default' style (paragraph) should be the rawest type of formatting
that the user wishes to use for her document.  Therefore, it should remove the
numbering and also set the 'default' paragraph style.