Power lost over distance?

Hi, I was wondering if a laser beam lost a significant amount of power traveling in the air? I’m asking the question because I have a flatbed like the one below and over the Y axis, at 0mm, I measure 20W, at 650mm (middle), I have 17W and at the end (1300mm), only 15W? It’s not a mirror alignment question because the beam is right in the center of the hole at both extreme position.

Note that I measure those powers BEFORE the third mirror therefore the only difference between the 0mm and the 1300mm is the amount of air?