sure, i agree, except that they also apply this 'narrowing down' logic when insufficient or no results are returned, thinking the query needs to be more specific in order to work. i have observed the following sort of behaviour:

    1. entering 'invisible marmalade teapot' and getting no results
    2. changing to 'invisible marmalade teapot with tartan cosy', again nothing
    3. so 'invisible marmalade teapot with tartan cosy in outer space', ditto

you get the idea...? it's just like in the real world, when you might go to a bookshop and say

    You: do you have that new crime novel in stock?
    Bookseller: er, i don't know which one you mean?
    Y: the new crime novel by john grisham, pelican something or other?
    B: oh, right, yes, here it is!

and everyone is happy.

default OR in a search engine would mean my first example eventually starting to return results about transparent space coffee pots with tartan cosies, ignoring the first few terms but the rest match, which is often helpful, particularly if you're doing an exploratory search for something where you aren't sure of the exact details.