After a deal is announced the companies involved usually trade at a discount or premium to the proposed acquisition price. This spread occurs for many reasons the main ones being:
The risk of the deal falling apart, usually factors include regulatory issues, shareholders against the deal, financing problems and more
The appearance of other interested suitors that may lead to a bidding war
The timevalue of money, the deal may take months or even years in highly regulated industries to close meaning a greater discount to the acquisition price