If the form you filled out where you checked that box was an actual FCC form, then it was the FCC that screwed up. Otherwise, it must have been one of the volunteer examiners that didn't transcribe the info correctly.
Fortunately, there is a way to fix this, and it won't cost you any money. First, you need to register on the FCC's Universal Licensing System (ULS) web site so that you can manage your license online. Not sure if you already did this. If not, the login page is here:
https://wireless2.fcc.gov/UlsEntry/licManager/login.jsp
There's a link below the login boxes that says "Register with the FCC." You need to click that to sign up for an FCC Registration Number (FRN) which will be your user ID to access the system. At some point you'll be prompted to set a password too. (I don't remember the exact process; I set up my account at least 10 years ago). Note that once you're registered, you should receive notification in writing from the FCC that your account has been created. You'll get two letters: one with your FRN and registration info, and a separate one with your password. Keep these for your records.
Note: you probably already had an FRN assigned when your license was issued. If so, it will be printed on the license itself. You may still need to set up a password on the ULS site though. (I got my license before they created the online system, so I had to apply for one directly.)
Anyway, once you're registered, you should be able to associate your license with your ULS account, and then you'll be able to manage your license online. Once you're completely set up, the first thing you should see when you log in is a "Your license at a glance" screen which will show your current callsign and license info. On the right there should be a box that says "Work on this license," and one of the options should be "Request Systematic Callsign." Do this instead of requesting a vanity callsign.
You should be prompted to fill out a form and then submit it for processing. There's no fee for this, as far as I know. You'll be assigned the next available callsign in the pool for your license class and the zone number for your location. It'll probably take them about a week to do it. You'll get a new license certificate in the mail.
You can use the ULS to renew your license, change your address/contact info, request duplicate copies, and so forth. As far as I know, there's no fee for any of these things, _except_ for requesting a vanity call.
-Bill