This is controlled by this code:
"and"=>", ", "or"=>" ", "start_encap"=>"\"", "end_encap"=>"\""
this says that any space is an OR and any comma-space is an AND
It also makes it so any search enclosed in quotes is treated as a whole phrase search.
so searching: this that
will return results for: 'this' OR 'that'
searching: this, that
will return results for: 'this' AND 'that'
and searching: "this that"
would return the exact match: 'this that'
You can change those settings. If you leave the and and or values blank then everything would be an exact match like:
"and"=>"", "or"=>"", "start_encap"=>"\"", "end_encap"=>"\""