There's a mistake many business people make when it comes to using contracts as firepower.
My mother and sister are both heavily involved in sales, and they both say that their lawyers are EXPERTS at writing contracts, whereas their client's lawyers are not always up to scratch.
This means that even if a sales person strikes a fair deal with a client, the terms agreed on may not necessarily be exactly represented by the contract, and usually end up favoring the salesman. In many ways, this isn't the sales lawyers' underhanded dealings - their job is to make the contract as profitable and airtight for their own company as possible, and if the client's lawyers overlook something, then it's their own problem.
So now you're the service provider and you've got a fat contract with huge benefits that your client has signed, sealed and delivered. It's legally airtight - you've effectively doubled the profit you originally agreed on with your client. So what do you do?
This is CLASSIC short term thinking, and what's worse, it's almost justifiable.
"Hey, they should have read the contract."
SURE, but if you want to
1) Retain an ounce of the integrity of your spoken word
2) Build a strong, lasting relationship with your client
3) Be recommended by your client to a slew of potential clients
THEN you IGNORE the contract and provide the service based upon your original agreement.
In the long run, you can only benefit.