Interaural Intensity Difference
This is quite simple and easy to simulate on either headphones or speakers. Simply having a higher intensity of the same sound coming from one channel to the other, localises it as such. If there is more of the same guitar coming from the left channel than the right, our brains will localise that as further to the left whether we are monitoring on speakers or headphones.
The difference in intensity between the sound reach your left ear and your right, isn’t simply a matter of volume either. You have to consider the fact that by the time it’s reach your right ear, the head has masked it as well, further distorting the sound in a way that is more tonal. This, as well as interaural time difference are theoretically reasons why monitoring stereo between speakers and headphones are very different.
Interaural Time Difference
This is fundamentally the reason why headphones don’t produce a true stereo image. In the real world, when a sound comes from the left, the right ear hears the same thing, just slightly later than the left ear does. Our brain uses this information (as well as IID) to localise the sound source. This difference in timing between left and right is known as interaural time difference. In headphones our left ear only hears the left channel and the right only hears the right. If you pan an instrument halfway to the left, the right ear hears it also but with no variance in time like there would be with speakers.
Via my blog post Stereo, Mono, Mid, Side, Panning, and Imaging Explained
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