If you search in JIRA for 'keyword1 keyword2' JIRA will treat is as 'keyword1 OR keyword2' and display more results than if you search only for 'keyword1'.

This is because Lucene's default operator is OR.

You can easily switch the default operator, making Lucene work exactly like Google does: the more you enter in the query field, the narrower is the results list, by including this single line into the code that does the search:

QueryParser parser = new QueryParser(DocumentBuilder.ALL, analyzer);
parser.setDefaultOperator(QueryParser.AND_OPERATOR);

Please consider doing it in JIRA. It will improve JIRA search significantly, at a very small cost.