If the “change currency” widget is never used, then it will use the main currency as the default one (in your case EUR if that is what you set in the General settings). However, once you use the “change currency” widget, it will remember the selection and keep showing that as the default. So it is likely you used The “Change currency” widget in your browser once so it is remembering your last selection.
Post a link to your product page so I can inspect it using a fresh browser that has never visited your site (so I can see what your new customers will experience).