I’m assuming the refs looking at each other is what they always do to make sure they make the same call. Undoubtedly it was close. My guess is that the right post ref was telling the other guy he thought it was a bit outside, so they ruled no good.
The only thing we know for certain is the kick started outside the upright, and it certainly ended up inside the upright before it landed in the net or the stands or whatever. But its impossible to know if it was inside the upright when it crossed the plane of the post.