When items like this go up for auction, sellers often set a "minimum" price. If that isn't met, the item isn't sold and remains the property of the original owner.
As you've highlighted, the reserve price doesn't have to be guaranteed by the auction house. So if this price isn't met, nobody gets the item (unlike the standard which is that the auction house will buy the item for the reserve price).
The 50% is probably the guy rounding the estimate to make a point.
As for why 64.25 wasn't enough but 70 would have been, I don't know. My understanding is that a lot of pieces sell for above their high estimate, so I'd guess that the seller was expecting/hoping for that? I do also think that 35% versus 40% matters a lot more when that difference is 6 million dollars in absolute terms.