I always wondered why teams play better at home than away, especially in champions league. Also, teams seem to play more defensively in away games. But what is strange is that despite this defensive mindset teams generally concede more goals in away games

If you don't believe me check the stats.
One example is Arsenal who are perfect at home but horrible away. They beat Braga 6-0 at home, but lost 2-0 away. Also, beat shaktar 5-1 at home, but lost 2-1 away. Someone plz explain how this happens.
Although one exception is Manutd who suck at home and are perfect away.
Is it the travelling makes away teams tired? Jetlag maybe?
Is it the home crowd singing 'you'll never walk alone' or chanting their crazy slogans that intimidate away teams?

Or does the crowd boost the home players instead?
What about manutd.....does the home crowd put too much pressure and make them play shit in Old trafford?
Are referees influenced by vociferous home support?