I am teaching myself Information Retrieval from Christopher Manning's book (PDF link: <http://nlp.stanford.edu/IR-book/pdf/01bool.pdf>). I tried Exercise 1.13:

"Try using the Boolean search features on a couple of major web search engines. For
instance, choose a word, such as burglar, and submit the queries (i) burglar, (ii) burglar
AND burglar, and (iii) burglar OR burglar. Look at the estimated number of results and
top hits. Do they make sense in terms of Boolean logic? Often they haven’t for major
search engines. Can you make sense of what is going on?"

By my knowledge of Boolean logic, the number of results should be like this:

burglar AND burglar <= burglar OR burglar = burglar

But this isn't so. In fact, on Google, it is:

burglar > burglar OR burglar > burglar AND burglar

So, what exactly is happening behind the scenes? Any pointers?

Note: This is NOT a homework problem, even though it is from the exercise of a textbook.