I was told by a developer that the database my add some more information to the people and group field in the list which I cant see and Nintex not showing. This way it cant match my variable, even if its the same people and group type value.
In my list I need to add one more column where I copy the IDnumber of the users in a single line of text field, than it will match.
I don't think it's a matter of adding and hiding anything.
if you make a query to sharepoint list and want to filter by people and group field you have to do so by Sharepoint's user ID.
here is some example, even though not directly nintex one, but I believe it might give you a good hint Using CAML to compare SharePoint "Person or Group" fields - Stack Overflow
so within nintex workflow, get SP's user ID (eg.) ith with set variable action and respective return type and then configure update item action to filter by that user ID.