So back in Feb. of 2018 I got and Equinox 800 and that was the first gripe I had. A penny and a dime ID'd to close together, same or over lapped. The Etrac or AtPro I had used never had a problem distinguishing the difference. Fast forward to the Dll, 900 Legend and so on. It seems that its a characteristic of SMF machines. I had hoped with the larger TID spread would help the issue with the Dll and 900/Manticore but it hasn't. A problem I run into along with that is most times a high conductor, penny/clad dime will ring in very high on the scale making you think you have a silver coin. I learned how to deal with it on the 800 and then the 900 but so far with the Legend a clad dime will hit 47-48 but still can't tell the difference between a wheat or memorial penny.
So to answer your question, no its not you, its SMF, and of course the conditions you are using it in. Keep in mind SMF has its advantages over SF machines but overlapping Id's seems to be one of the trade offs. Good luck with your Manticore.