yeah my explanation was very simplistic and ignorant honestly. Thesituation is onto something.
Found a nice report on the differences.
"3) It is interesting to note that the major muscles involved (pectoralis major - sternal portion, latissimus
dorsi and teres major) are the prime movers in either the supinated or pronated grip. However, only
the upper latissimus is involved when the pronated grip is used and only the lower latissimus with the
supinated grip. Thus by using both grips you can get full development of this muscle while still
involving the others to almost the same extent."
"When a fairly wide pronated grip is used and the back of the neck is brought up to the bar, the
latissimus dorsi muscle pulls in a straighter line and exhibits greater force. The lower pectoralis major,
however, is favored when the chin is brought above the bar when using either grip."
http://kinesconnection.com/exercisepdfs/Chin-upPull-up.pdf