I think it would be safe to say "buyer beware" when it comes to variation of quality on their Signature line.
Do check out those images they give you on the GCAL cert, because on those certs will be the only imagery you can obtain from BN.
I don't want to come off as scathing to BN, but just as there are many many good stones on BN there are also many that are less than optimal. Unfortunately, it seems like their Signature line is not immune from this.
With BN, going with an AGS graded stone or paying close attention to the certifications is the way to go.
I'm sure their Signature line is solely based on paper parameters and not a visual selection. If on the off-chance that it is based on a expert picking out true 'gems' among thousands, perhaps it can be an example of one of those diamonds that would score poorly on the HCA or other metrics, but on the other hand is simply gorgeous in the eye of the beholder.