When an architect is not involved to determine the extent of the services needed to repair a building, the contractors are left to determine the scope of the repairs by themselves. This frequently results in vast differences in their bids and the work that will be done for the money. Many times there is confusion as to what actually needs to be done.
There are companies that are writing their bids in such a way, that it appears that far less work needs to be done than in reality, so as to have the lowest bid. Then when the work is in progress, they either neglect to do as much as is necessary, or they remind you to look carefully at your contract when they send you the bill. There is an added charge for work they determine should be done on the fly. These surprise costs can be painful. Suddenly, the company is no longer a good deal, and long-term that may become more and more apparent.
.No one can really know the extent of damage that lies underneath the surface before they tear in. Outside of catastrophic unforeseen damage, which may need a plan B to be determined, there will not be added costs.