Because of the price. The confirmation that was sent was priced for 7 days. Disney's system showed 7 days. When the OP went to link the reservation to
MDE it showed 7 days. Everything points to 7 days except perhaps for the dates on the confirmation. But even if that shows 9 days, if the price shown is for 7 days then the buyer owes the difference. If Disney offers a package priced at $340 per day and the buyer wants 9 days, then the buyer owes 9 x $340. If a mistake is made and the confirmation shows a price calculated based on 7 x $340, then the buyer either accepts the deal for fewer days or pays the difference to bring the deal up to 9 days. Look at it like this. You go to the grocery store and place 4 apples in a clear plastic bag. The price is $0.30 a piece. At checkout the cashier initially thinks that there are only 3 apples in the bag and rings up $0.90. As she places the apples on the bagging belt she notices that there are actually 4 apples. At this point you have a choice. Either remove an apple so that the amount you buy corresponds to the price paid or agree to pay $0.30 more. You don't get to say: "gotcha! You only saw 3 so I get a free apple". Now, if the apples had been mispriced at the point of sale and were offered at 4 for $0.90 and the cashier tried to say that the price was wrong and they were really 4 for $1.20, then you would be within your rights to get the apples at the lower price. Indeed some state laws mandate that result. But here I have seen nothing that suggests that the unit price was wrong. Every system reflected 7 units at $340 per unit. When the confirmation came in and showed a price that was $681 shy, then the OP should have known that only 7 units had been charged and that there was an unpaid apple in the bag.
Look at it another way. You want to buy tickets only. You know that they cost $350 each. You place 4 in your on line shopping cart. At checkout your total shows $700 because of a glitch. When the tickets arrive in the mail there are only 2 in the envelope. You call Disney and they tell you that their system shows that you only bought 2. You tell them that your screenshot shows 4. The CM says that tickets are $350 each and that if you want two more you will have to pay $700 more. Pretty simple. Do you really think that Disney was offering you a contract to buy 4 for $700?
Now flip this so that in the example the computer showed a price of $175 per ticket. You buy 4 thinking that the total should be $700. Under this scenario, I side with the buyer (and so does the law).
To me it all comes down to whether the unit price was misrepresented (in which case the buyer wins) or whether there was a mistake in the number of units the buyer and seller thought was part of the transaction (in which case an adjustment is warranted).