Why does listening to music on my computer sound better than my iPhone 4? - 1

Today, while I was browsing iTunes I decided to listen to some samples with my earphones. I own the regular Apple earphones w/ mic (the ones that come with an iPod). The sounds from my computer had a surprising quality, compared to my iPhone 4. For instance, it sounds more HD, the bass is so much better, etc. But when I play the songs on my iPhone 4, there's hardly any bass and it almost sounds like my earphones are broken when they're totally fine…

Why is this?
Computer/iTunes (High quality sounds w/ bass) VS. IPhone 4 (Poor quality sounds w/ hardly any bass)…