I am sure this is how they do this. I cannot think of any other way. Disney controls its pricing TO THE CENT. If any
travel agency offers you a discount that Disney itself is not advertising, I would inquire as to where it's coming from, so as to avoid a possible scam. I'm a little surprised that Disney would allow an agency to purchase room blocks and then advertise it as discounted rooms.
In fact, I'm willing to bet that Disney might not know about it? I can't imagine Disney allows travel agents to undercut its own pricing.
Disney absolutely knows they are doing this. They want to fill rooms and are willing to sell them "at bulk", so to speak, to fill them. Plus, the only way to book these rooms is to call Disney groups, so how would they NOT know? No one is "undercutting" Disney pricing. The pricing is being
set by Disney Groups, through contracts, and all the money is going straight to Disney!
There are quite a few agencies that contract with the Disney Groups Dept. (as a convention would), get the room blocks at a discount and then pass the discount on to their clients. Boscov's (the department store)
http://www.boscovstravel.com/ used to be the biggest Disney group sales agency in the US, but I don't know if that's still the case. If you look on their website, you will see their exclusive charters advertised. This is all legit and contracted through Disney.
As to why all agencies aren't doing it? - my guess is it's because negotiating the contracts takes a TON of time and effort. If anyone here has ever booked or managed a convention or large group (wedding, etc.) at a resort, you know what I'm talking about. Now multiply that experience with the number of travel timeframes, per year, you want to offer a room block. It's a ton of work. It's also less money, in the long run, as the TA's will get less commission on the discounted room than they would get if they booked it at rack or a general public discount and there is no commission on the tickets. So agencies really need to do a cost/benefit analysis to see if entering into these contracts will ultimately be of any value to them - or just a huge pain to manage with little reward.
A suggestion - if you question an agency, look to see if they are "EarMarked" (they will typically have a special Mickey logo on their web page showing their status) or check here:
http://www.authorizeddisneyvacationplanners.com/ Check their FB page, look at yelp reviews. We live in an age where a simple google search will let you know, pretty quickly, if a business or business person fulfilled their promise to their clients.
Third-party wholesalers (like Expedia and Orbitz) also frequently offer discounts beyond Disney's general public promos. These sellers work through the Disney Wholesale dept. and are also completely legit.
So, sorry, but, you lost your bet. Disney has been entering into group contracts with travel agencies for years.