There's a difference. Have you played high school or college soccer? You can tell the difference even when you play in a small stadium with barely anyone watching. It's the feel of your home field and the fans cheering you on. Teams also tend to go into away games with a defensive mind-set which often hurts them, but is seen as necessary due to the disadvantage of playing on the road.
Mourinho hasn't lost a home game in 9 years, so there's definitly a lot to it.
