It was written somewhere on the SCP website that a lot of the earlier SCPs weren't as standardized in their categorizations. I agree though this one should probably be safe, since you could lock this in a locker, throw away the key, and it would be more or less contained.
From: http://www.scp-wiki.net/object-classes
If an SCP is very dangerous should its Object Class be higher?
No, danger does not really affect an SCP's Object Class. As has been reiterated several times above this, an item's Object Class is more based on the difficulty of containment rather than the danger it otherwise poses. For example, a button that can destroy the entire universe when it's pressed would be safe, whereas a cat who randomly switches places with another cat anywhere on earth would be considered Keter.