My first reaction was "That can't be right, we have never been to the USA. This must be a mistake from geocaching.com. Just like the California-souvenir we received in september 2013." But my other half said that he did solve a couple of assignments, got bored with them and stopped doing them. So, finally the problem is solved.
I just have to live with a discrepancy in finds on geocaching.com and GSAK.
Cerberus1 and Kunarion, thank you so much for your help.