I think it's a generic setting that Ebay uses for the Seller's protection. If they have more than 1 item of that kind, they don't want the subsequent Buyers knowing what the others paid.
I've had multiple items that I've sold under Best Offers; sometimes I get more out of the second Best offers than the first.
Strange... never seen that before. Sucks for people doing value estimates! I'm soooo curious!