The time delta can all be explained by the cruise speed (to a rough approximation).

Figure 5000 miles at cruise. That would be

8:55 for the 777's
8:30 for the 380
9:25 for the 767

So DL would be expected to take almost an hour longer than BA, and 30 minutes longer than the others. And that is what we see.

Yes, that gets tweeked by TOD, wind speeds and gate locations and whatever. But no indication at all here that DL is over padding relative to the others.