Callus come in all forms but this one results after repetitive motion friction-trauma of the same spot of skin, albeit the base of the left or right palm. Of note, if you have this callus you're also at greater risk of carpal tunnel syndrome (CTS) because you are playing with your wrist flexed (dorsi-flexed). You do not get this callus or the CTS if you play with your wrist in a non flexed position. Here is a beautiful example of the right hand callus.
I just could not help myself when I found this at TL.net. If you have similar lesion take a picture and send it in.
Links
TL.net - Source