Computers require code to operate, and simple commands, such as adding a line that places data after the program determines a certain action was completed, can change an output. Otherwise, simple code could place a set vote for a candidate every 5000 votes, and it would probably never be noticed. Regardless of the vote, the computer counts the vote for the preferred candidate.
It should be simple to examine the code in the voting computers. That, and questioning the programmers on what they programmed. Considering the simplicity of this task, I wonder if there are some programmers being grilled at this point.