NH 8x DIA and MM here... The alliance with United really screwed up ANA's pricing.
Back in the day, you were able to hold advanced bookings for ~7 days without paying. The pricing scheme was also more simple and predictable. Essentially, the earlier you booked and more seats were available, the cheaper the pricing.
Since UA's alliance, however, pricing swings as with UA and other U.S. carriers and depends on a confluence of factors, routings, supply/demand comping with codeshare flights, etc. etc. Really awful. You would never have Business Class tickets drop $2,000 after booking. That's just crazy and insulting. Even if they can't do anything for you, you should call ANA and share your displeasure.