I've never seen an overall grade that is lower than any of the sub grades. As said it is usually either the lowest sub grade or in rare cases slightly higher (it is supposed to represent the overall appearance, eg a tiny crack may bring the bubble score down but not ruin the appearance of the whole piece.) But I don't see how that logic could be applied in reverse!
If the sub grades are accurate my advice would be buy it and crack it out of the case. It will probably sell for a low price and be worth more ungraded, such is the stupidity of AFA collectors. Some of my best scores have been 'low' graded AFA MOCs that I've released from their tomb. I once picked up a AFA 75 12 back Jawa for £55!