While there is some padding, the cruise speed variation is factor that adds time, plus the B763ER DL uses will be weight limited as it's close to max range.
Originally Posted by
TheMadBrewer
IIRC, NZ, UA and AA all do 777s with a typical cruise speed of 560 mph (according to wikipedia)
BA does 380 with a cruise of 587 mph
DL does 767 with a cruise of 530 mph
I don't know which variant A340 VS uses but both have a higher cruise than 767
Flying 5456 miles at cruising speed (simplifying things) you'd expect a 777 to do it 30 minutes or so faster.
That's not to say that Delta might pad more than others, but I think the plane does have some impact on the times...