IPhone Audio Output vs. MacBook Pro

Question

I've been the owner of the Apple In-Ear Headphones for a while now, and have experienced an improvement in the sound quality between the in-ear and standard iPhone headphones.
However, I've recently been using the same in-ear headphones with my MacBook Pro, and noticed a significant difference in the quality, depth, and volume of the sound. When playing the same songs on my MBP, the headphones are capable of much louder volumes, much clearer audio, and are much bassier than when using my iPhone 3G S, yet the audio tracks are identical.
Is there any specific reason why my iPhone isn't capable of matching up to the quality of my MBP?
Many thanks

Answer 1

I wanted to chime in after my latest show-down between my iPhone 5 vs. iPad 3 vs. MacBook Pro 2010.  My medium of choice is vinyl for hi-fi listening, but otherwise I prefer Spotify @ 320kbits.
I plugged them into my hi-fi system: rega brio-r amp, wharfedale diamond 10.1 speakers, standard [decent quality] 3.5mm to RCA cord to plug in each device.  No DAC in my setup. No cable splitting-- all directly plugged in.
I used the latest Arcade Fire album, Reflektor, 320kbits from Spotify.  I think the album has enough dynamic range to make it a good test subject.  I started the test on track 2, that song in particular having a lot of dynamism.
The breakdown:
Iphone5 and iPad3 both sounded identical.  Everything sounded good on first pass until I plugged in the MacBook Pro.  The MacBook revealed an enormous difference in better audio quality!  The MacBook output crystal clear highs and the lows also had a "thump" that the iPhone/ipad did not have.  That difference right there showed that the MacBook is simply capable of handling a higher spectrum of sound.  With a higher spectrum, and the ability to differentiate the highs, mids, and lows helped give the sound much more dimension.
Bottom line:
I wish I could turn back the clock on all those hours I spent listening to my iPhone.  Without a doubt, the MacBook Pro has a much higher quality audio output.  I think this is attributed to the fact that the MacBook actually has a SOUNDCARD while the iPhone/ipad both have like integrated circuit boards.  It's the dedicated soundcard on the MacBook that gives it its advantage. 
To break it down to hi-fi thinking, when a stereo has separate modular components that are dedicated to doing 1 thing well, there is a better opportunity for higher quality sound.  That would be in comparison to an "all-in-one" unit that crams a lot of electronics into a single "stereo" system.  Generally, the overall quality of the "all-in-one" will be hampered by what are basically the limitations of engineering and production; each new component/feature they cram on the same circuit board compromises the overall integrity of the whole.  What I'm trying to say is that the MacBook is more like a modular setup with dedicated components, and the iPhone/ipad are the "all-in-one".