Performance review skipping the approval process

Content

Hi,

We are in the middle of our performance review cycle and today we noticed that a couple of reviews, all under the same review manager, when completed, have skipped the approval process and went directly to the finalized stage.  I've checked a number of possible causes for this including ensuring that the approving manager was set up as a review approver and can't find an obvious answer to this.  We've completed 200 of 400 reviews and this is the first issue we have seen like this. 

Anyone have any ideas as to why this would happen?

Thanks,