Others have answered how to get what you want, but you also asked why, so here goes:
Your query will look something like this:
SELECT * FROM cars c
LEFT JOIN passengers p ON p.car_id = c.id
WHERE c.name LIKE '%banana%' OR p.name LIKE '%banana%'
Basically, you're joining cars to passengers, and there are three passengers in a car where the car.name is 'banana'. So that returns three rows.