During the development of the Boeing 757, once it was decided that both the 757 and 767 flight decks were to be common, the original design for the 757's cockpit that was based on the 727 now faced the challenge of having to fit the widebody cockpit of the 767 onto the single-aisle design of the 757. In a rare move for Boeing, the design of the 757's nose was reconfigured after program launch in an effort to achieve a common rating for both aircraft.
Using the same forward windshield, related structures and cab geometry as the 767, the design was then altered to suit the narrower fuselage of the Boeing 757 with the goal of having the same visual and ergonomic geometry for the flight crew as that of the larger 767. The 757's nose section curves upward more dramatically than past Boeing jets as the point of nose is actually below the fuselage centerline with the flight deck floor a step down from the main deck.
The result was a cockpit with more visibility than past Boeing designs and aerodynamic noise also improved, with a 6 dB drop over the aerodynamic noise of the 727. The wider cockpit improved storage space with more room as well as improved air conditioning circulation patterns and the by providing a wider space at the front of the aircraft, there was more space for the forward lavatory and galley.
The extensive changes to the 757's nose and cockpit were all driven by the need for a common type rating for both aircraft and as the 767's cockpit had been designed first, it was adopted for the 757 with modifications. But with an identical windshield and instrument panel, much of the associated structure was also nearly identical in many ways.
Source: Boeing 757 and 767 (Crowood Aviation Series) by Thomas Becher. The Crowood Press, 1999, p33-35.