go take a look at the Red and Green Channels in Photoshop and you'll actually see some stepping and a gradient. Particularly the Red channel. Since your texture is just flat surfaces with divots and bolts etc most of your normal map should be RGB: 128,128,255. In normal map terms that color is "flat" and wont have…
Its possible the HP was not completely flat when baking. For instance if you only had holding edges at the bevels but no additional edge loops throughout the flat parts of the model you can get a very subtle arch across the "flat" surface from bevel to bevel... I think. This looks like it was modeled and baked as a flat…