Hi Ryan,
This is indicative of the campaign member records being unavailable in Marketo. This has two typical causes. First, the campaign was recently created, and the campaign member records have not had a chance to sync down to Marketo. Second, the campaign or campaign member records were not visible to your sync user at one point, and then later became visible. This required a SystemModStamp update to the records in SFDC, at which point to Marketo batch sync will pull them down, assuming that they are now visible.
There are other potentiall causes, but they are significantly more complex.