Hi @dev2EP8UX,
I hope this message finds you well. No need to fear, the reason for the difference is easy to explain. The polygon implementation is completely different in Fusion compared to EAGLE. In EAGLE the polygon width is used to create a raster to fill the polygon. In Fusion, we fill the polygon with full resolution so there is no raster.
In EAGLE, when you draw the polygon edge you are defining the centerline of the thickness as the polygon edge but it actually fills half a width extra beyond the centerline. When you import the EAGLE files into Fusion we want the fill to be the same so the values change to make sure the polygon fill covers exactly the same area as it did in EAGLE.
I recognize the above is very verbose but the short version is that the values change because of the difference in how polygons are handled in Fusion.
Let me know if there's anything else I can do for you.
Best Regards,

Jorge Garcia
Product Support Specialist for Fusion 360 and EAGLE
Kudos are much appreciated if the information I have shared is helpful to you and/or others.
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