Sellers are neither nasty nor nice; both sides are negotiating for their own advantage. Fixed price transfers all the risk from the buyer to the seller.
When a contractor bids a fixed price job, it has to make a series of assumptions about what its direct costs to complete the job will be, what portion of its fixed and variable indirect expenses should be allocated to the job, and what amount of profit, if any, should be included in its bid price, taking into account the competition it faces, and its need for the work. In theory, the contractor is either given, or is able to obtain, sufficient information about the job, and the restrictions under which it will have to be performed, to enable it to make correct assumptions, and to accurately estimate what its costs will be.
There are many times, however, when a contractor’s cost assumptions are based on incomplete information, or are based on reasonable but mistaken beliefs about what the owner will do to facilitate the performance of the work. When that happens, the contractor often incurs substantial additional costs that it did not anticipate, and wants to submit a claim for additional money to the owner. mcsmag
The seller's best option is to plan thoroughly and cleverly, and this is what both parties are hoping that the seller does; the price negotiation is, in part, the two parties agreement on the quality of the seller's planning. If the planning is done right, all the possible exigencies will be covered, and the work will be completed in a way that matches their mutual expectations about quality and profit.
But given that all planning is imperfect, when the unexpected occurs, the consequences have to fall on someone. Except in rare occasions, neither side is enthusiastic about claims - but sometimes claims are a rational, prudent response to exigencies.
"Courts have also recognized the risk-shifting mechanism of fixed-price contracts. “Unlike the cost-reimbursement type contract in which the government bears the burden of all allowable costs, the burden is shifted entirely to the contractor in a fixed-price contract,. . . ” jdsupra
Aside: the case law referenced in the above article contain multiple examples of the buyer attempting to use "claims" to claw back money from a seller whose superior planning enabled them to make an unanticipated profit. These claims appear to be denied.
Fixed price/lump-sum contracts In this type of contract, the seller assumes the greatest risk because the price is set. This means that the seller must comply with the contract and provide the service/product for a specific price. If the time it takes to deliver the service/product expands, the seller cannot charge for the extra time. The same would be true if extra materials were needed to deliver the service/product. If this occurs, the seller cannot charge for the extra materials because the price for the contract is already fixed. flylib.com