Was does a tag field value "ID3v2.3 (ID3v1 ID3v2.3)" mean?

Yes, that means, both tag sets exist in this file and you are currently watching the one which is displayed in front of the patenthesis.

Check File > Options > Tags > Mpeg to controll which tag versions you want to be displayed and written.

More on that here:
/t/965/1
https://docs.mp3tag.de/customization/options/tags/mpeg