Flechette ammo deals around 6 damage less per bullet in close quarters than buckshot. This means that you can be less accurate per shot with buckshot (more bullets can miss) and still do the same or more damage.
Flechette rounds pierce targets better though, so if there are 2-3 targets in a row, you'll get more overall damage per shot, but still less damage on the target in the front.
At maximum damage dropoff, flechette rounds deal about 3-4 more damage per shot.
It all comes down to the fact that because you need less bullets to hit a target in close range with buckshot, you can actually 1 hit kill from further away, as with flechette rounds you need to be close enough to have nearly every bullet hit. Although this is slightly mitigated by the fact that fletchette rounds have a slightly smaller spread, meaning if you're accurate enough, more bullets should hit at range than with buckshot.