Essentially, the lowest Amtrak fares are sold to those who make the reservations first, with fares rising as the train begins to fill up. Hence, the highest fares are paid by those who make last-minute reservations (you are always quoted the lowest fare available at the time you make the inquiry). Basically you guessed correctly, since more people are taking the train the lower priced seats have probably already been sold, producing the higher fare levels you found.
It's going to be really hard to beat the Many Happy Returns (H208) promotion. Amtrak has many discounts available, but they cannot be offered unless you know to ask for them. Unlike WDW discount codes, there is no listing of Amtrak discounts for you to check. You might try Northeast-area newspapers, and the Amtrak website does have a search function for some specials, but there is little there to benefit you right now except H208.
Incidentally, this morning Amtrak showed a fare of about $536 for your travel dates, so snag the $385 if possible. The 15-day East Rail Pass goes for $260 per person during this time, which just beats this mornings quoted fare.
