My Astral XX, also manufactured by X-treme has the center seam as well. However, it never affects the performance of the plane, and will not deter me from buying another one. However, X-treme did manufactured another version of the XX with no visible seams named the Aurea for Japanese market only.

If you want one with no seams, you can get the all white version, sand the seams down and paint it yourself.

Adrian