The weights fall under gravity, accelerating downward—that is, both strings go faster and faster as they fall. Specifically, each weight falls a distance proportional to the square of the time that it falls. As a result, String 1, with its equally spaced weights, hits the surface with shorter and shorter time intervals between the weights.
In order to hit at equal time intervals, the weights must be spaced so that their distance from one another increases proportional to a square. If you look at where the weights were placed on String 2, you’ll notice that the distances between the weights are proportional to the squares of the number of each weight in the sequence: 1, 4, 9, 16, and so on.
The weights on String 1 are spaced at equal distances. Let them fall, and you can hear the way objects accelerate under gravity. The weights on String 2 are spaced to equalize the time it takes for each weight to hit the floor, so the rhythm of the weights hitting in free fall is nice and even.