SSN and ITIN will always be different. They are issued by different agencies. SSN is issued by social security for the purposes of recording your payments to the Social Security system. Since it is a unique identification number, it has often been used by other agencies for primary identification criteria, such as the IRS. When the person does not have a SSN but has income that requires they file a tax return, the IRS uses a unique 9 digit number for their primary identification (the ITIN) for income tax purposes. If the individual later gets a SSN, it will be a different number and they should no longer use the ITIN to report taxes.
Many companies have used SSNs for identification including the CBs and SSN is just one of the identification methods to make sure they have the correct person. It doesn't surprise me that they might use an ITIN in the absence of a SSN, as their primary purpose is to identify an individual with his/her correct credit file and sell that info to creditors. Creditors can certainly issue credit without a SSN (or ITIN) if they choose to, but may sometimes have certain policies against it. It may cause some issue to try to change over to an SSN when issued, but they would probably figure it out and list as aliases or known variations used. Ever notice that field on your CRs?