At a glance, it looks to me, like the piece of leather they used for the bottom part of the spat, seem and all went out up to the top, then they put that top band over top of that. In other words that line on the top band looks to me like there is a seem under it, but not a seem there in the top layer. (if that makes any since, it did in my head. :lol)
Either that. OR, because there is a seem on the bottom part of the spat, that would be a natural folding point for the boots when not on. So when Nathan is not wearing them, and they are just setting some were they may have folded along the seem on the bottom have, and that fold may have left a small outward crease on the top band.
Either way, it does not look like there is a seam there, on the top band. To many anyways.