That looks like yet another bug in the online booking tool - it appears to be pricing your second itinerary as a DONE4 (although even then $500 tax on the $9100 base fare seems a bit low when the AONE4 has $1000 tax on the $12500 base fare for much the same itinerary). As you noted it should be a DONE5, for which the base fare ex-USA is $10400, so the all in price will be at least $10900. I guess this would get spotted once the quote leaves the online system and goes to a real ticketing office for confirmation.