It is ultimately the fine scratch texture. It is both holding onto some grime, and you are seeing the texture difference. The finish is quite thick in most cases. It will look better depending on the strength of how you clean it. Generally something like a magic eraser is pretty good. However, the best will come from polishing. It does not have to be anything elaborate. A machine polish with a heavy cut pad and compound like I would used to cut and buff automotive clear coat would be overkill, but are an example of what I mean. If I could get a polisher in there, it would take me 5-10 minutes with a cut pad and 3M PerfectIt 2. Toothpaste is technically a polishing compound although only around a medium cut. That with the inside of a sock as a polishing pad for an hour or more by hand will get nearly the same result. Keep the surface wet like a wet clay that feels barely warm the whole time. Never let it get dry or wetter. Keep the patterns random circles and never repeating rows or columns; keep it more random than that. Do something like a puffy cloud that is 2/3rds the total size moving around the surface. That will make it good as new… if you are able to defeat your inner impatience.