There can be a lot of factors involved in that, though. The amount of aircraft in the sky is really high, and accordingly flight paths are often fairly indirect with inefficient corners involved to make the most highly de-conflicted flight paths possible.

When your plane is extremely delayed, to the point that you're the last flight taking off from your airport (and thus likely a large set of local airports), this makes the likelihood that a huge amount of airspace is more clear around your flight path now than it was for the originally scheduled flight. The updated flight path might be massively more efficient, hit favorable wind conditions, open up different speed options along the way, etc.

Since flight times are dependent on a large amount of conditions that are pretty volatile with respect to time, the original estimate was probably true, but with changing conditions and flexible scheduling, they managed to get you where you were going on time.

So, sure, maybe the 'flight time' isn't true, but it's a pretty clear indication the arrival time is.