Dept. list missing values in Requester's record

We need to manage a number of Departments (about 50) in different locations. I've created about 20, and then imported users list from a CSV file, including a column with the appropriate Department name.

All went fine, but now I'm facing a strange behaviour: if I create a new Requester, the Dept drop-down lists just a part of the complete Department list (14 ?).

More, if I delete one Dept into the appropriate section of Administration, th drop-down list kind of "pops out" one of the 14 listed, adding one that wasn't shown before.
Since we are deploying the solution to all of our Depts., we definetly need to assign the proper Dept to user to correctly set Biz Rules up.

Could you please have a look ? It's becoming urgent, since we are deploying to about 600 users from 4 Depts. ...

Thanks