Administrators do, by default, get access to all standard fields on all standard objects. There's a number of reasons why this field might not have had visibility for administrators. For example, removing a field from a page layout in Professional Edition used to also turn off field level security, as they didn't have custom profiles. If you later upgraded to Enterprise Edition, that Field Level Access would still have been turned off.
Or, maybe an administrator figured nobody was using the field, so just disabled it for everyone, to clean up reports, layouts, and list views. Or maybe it was an accident, and didn't realize what they were doing for some reason.
There are plenty of legitimate reasons why you might want to turn off field access, even for administrators. And administrators can generally do whatever they want, even potentially doing things that could remove their ability to log in!
And, because of how the security policies are at salesforce.com, there can be quite a bit of red tape involved fixing these sorts of problems. Administrators do have the authority to do pretty much whatever they want to do, even some things that would be really bad.
If the change was done in the last six months, the Setup Audit Trail should show who removed the field access, and when, but aside from that, all we really have is speculation. Someone, somehow, decided to disable this field, for the administrator profile.
Even though System Administrators can do pretty much anything, they can also restrict themselves from doing things as well; the only real restrictions in place on administrators is that they cannot revoke their own administrator permission (e.g. they cannot make themselves a Standard User, or deactivate their own user profile).