What you want is
Code:
title:life title:pi
not "life pi". Incidentally, if you search "life pi" in google (with the quotes) it does not match the string "life of pi" either. Instead it matches a bunch of pages that happen to have life<punctuation>pi in them (look at the part of the search results in bold).
Interpreting quotes as denoting an exact string match is standard practice in pretty much every search engine I have come across. The only difference is google seems to interpret a space in a quoted string as being "any punctuation" which calibre does not.