The 747-based Air Force One (VC-25A) uses the livery that was first used on the 707-based Air Force One (VC-137) shown below (wiki commons photo).
The polished aluminum area that you observed on the 747-based Air Force One, appearing as a "swoosh", can be seen as it was originally designed for the 707 AF1 (VC-137), with the polished area more clearly continuing all the way to the tail of the aircraft.
On the 747 AF1 the polished aluminum is interrupted by the painted aerodynamic wing-root fairing, making it appear as a "swoosh".
As a result of aerodynamics expertise increasing toward the end of the 1960s when the Boeing 747 was being designed, Boeing designed an aerodynamic fairing to smooth the turbulent airflow at the wing root where the wings attach to the body.
During the development of the 757 and 767 in the very late 1970s Boeing decided to utilize composite materials in order to save weight on light aerodynamic components like flight controls and fairings... eventually Boeing applied this weight saving technology to the wing-to-body fairing of the 747.
Because composite components cannot be polished to a shiny luster like aluminum can, Boeing paints these parts grey.
The "swoosh" on the 747 AF1 is a result of the polished area (which is uninterrupted on the 707 AF1) being interrupted by the painted composite wing-to-body fairing.
Regarding the 757 (C-32A) livery which lacks the polished metal aspect of the 747, it may have been omitted due to the C-32 not having the specific primary role of transporting the President.
The C-32 is intended to carry the Vice President, the first lady, and the secretary of state. Other members of the president's Cabinet and members of Congress have flown aboard the C-32A, as has the president when traveling on short domestic trips where the range or capacity of the 747 is not required or the runway is not long enough to accommodate a 747.
Perhaps the most likely explanation for the 757 not having a polished metal area is that it takes too much manpower to keep the bare metal areas of the two 747s polished and looking good for the cameras.