Ok, I'll reveal the solution of the riddle.

A position-dependent flash sequence can have two reasons. The simplest one is that it's a very old airport with rotating beacons for the runway lights. All beacons rotate synchroneously, so their light beams always have the same angle. Therefore when looking at the runway sidewards from a distance, they all flash at the same time. However when an aircraft approaches the runway or takes off, the beacons are seen in slightly different directions by the pilot, so their light beams become visible in sequence.

However, our airport doesn't use rotating beacons but xenon flashes. Still, you see them flash in sequence even when they all light up at the same time. The reason is indeed the optical effect mentioned by Blattsalat. Our optical nerves react faster on a bright flash than on a dim flash, so close lights are seen a little earlier than far-away lights. Although the flashes appear to have the same brightness, it's an optical illusion - a flash close to us has in fact four times the brightness than a flash in twice the distance. For a similar reason, when you see street lamps suddenly go on in the evening, you have the impression that they are switched on in sequence.

BTW, this was a small hypothetical airport - big commercial airports use non-flashing runway lights and lights that flash in a real sequence _before_ the runway.