DL picked up the 4 ex-ATA aircraft (which were built for SQ and operated by them for a rather short time) because they were PW powered and available....I think that DL got the 4 aircraft in the late 1990s, when DL (and the other carriers) were expanding quickly and were happy to get any available aircraft. The Shanghai 757 is the other 757 that DL picked up around the same time......I believe that DL purchased it off the Boeing assembly line.
DL also purchased a few (6?) ex-Gulf Air 763s to expand international services and get the L1011-500s off transatlantic routes a bit more quickly.....these airliners had the right engine, although the ex-Gulf Air planes have a different cabin plan due to the fact that the emergency exit locations and pax doors are in a different config on these aircraft.
DL also picked up some 733s from Germania, on lease, to supplement the 733s that DL inherited from Western.
Times changed dramatically this past year, there are now so many planes parked and in storage.......but in the late 1990s, airlines were constantly on the look out for aircraft that would work within their fleets to allow expansion on a quicker basis.