Why do trails (supposedly) render with 1 less triangle compared to beams?

Someone told me they noticed that having many trails was significantly less performance impactful than having many beams. I was confused by this because I thought that a trail and beam segment were both only 2 triangles. Yet, when testing the draw counts for a trail and beam segment, it appears a trail segment somehow only has 1 triangle despite the wireframe clearly showing 2.
A single beam segment (2 tri):


A single trail segment (1 tri):

If anyone could explain why this is, I’d greatly appreciate it. Thanks.