Why the early ones are easier
A 03:30 departure sounds worse than an 05:15 one and is considerably more predictable. At that hour the M27, M3 and M25 are empty, the village is empty, and the drive takes exactly as long as the map says. By 06:30 on a weekday none of that is true.
It is why the 07:00 rows above have a smaller total allowance than the 09:00 rows despite the same check-in requirement. You are not buying time for the airport, you are buying time for the traffic.