If you're talking about the lighter area surrounding the map (presumably the transparent area of the decal), I'd say the finish (you don't mention what you used) interacted with the wood to come out looking darker/amber-er, but the area under the decal was protected and stayed light. It might not happen if you put an even coat of finish on the whole blank, then applied the decal to that and finished over everything. Unless that's exactly what you did, in which case I'm out of ideas.