My car won't start so I take it to a mechanic's shop. He tries the key and hears the starter click, but not turn over. he tries the battery to see if there's enough charge, which there is. The mechanic tells me he thinks it might be my starter, but he won't know for sure until he gets to it, and then gives me an estimate based on that opinion.
If he calls me up an hour later and tells me that in addition to the starter I also need a new ignition coil and that's going to cost extra. Am I supposed to get belligerant that he didn't see that during his initial process? Of course not. He told me (or I already knew) that his initial opinion was based on a process that had some limitations, and that a subsequent and more comprehensive analysis might reveal more information.