I had the same problem, some covers showing a Personal label in a white stripe.
On those books, in Calibre, I edited the Metadata to remove tags (they all had tags), then ran Convert to Mobi, even though they were already Mobi.
After deleting the offending ones from my Paperwhite, I Sent to Device from Calibre again and every time they came up showing the full cover, no Personal label.
So I'd guess the problem is simply down to the Kindle having a problem with tags, and Convert to Mobi gets you a new version without tags and without the Personal label
