Because Shirley is Head Judge so she has the casting vote - her vote carries more weight. Even though it was 2 - 2, because Shirley voted for Mike, then Mike stays.
Felt very sorry for Catherine, she looked devastated. However, it wasn't a very good dance. She is tall with heavy legs compared to her top half and this shows in her latin - she just didn't have the necessary speed and precision.
Did I hear a much louder cheer for Chris than anyone else, when it was announced he was safe?