If we talk about the mistake on your part, i. e., timing - yes, you have to adjust the grading. It is not 'somewhat' unfair, it is very unfair to have different exam duration from what was announced. Students have different strategies for answering exam questions, depending on the time available, and that could impact their rate of progress severely.
If students on average do too well on the exams in accordance to the established criteria, well, they were well prepared for the exam. You can make next years' exam a bit harder, or shorten the time if you think it was too easy - but it's unacceptable to change the grading curve because average is 'too good'. Students study and prepare for the exams, no need to punish them for doing well.
If on average the students did badly, curving the exam results depends on the cause of the low average grade. Are the exam expectations realistic? Did the students have enough time to read and understand the question, think, and write a sensible answer? Remember, stress slows people down + sometimes the questions are not clear. Were the questions too in-depth and missing the more important, general aspects? Were the lectures helpful to learn - were there materials/excersises provided? Were all the questions formulated in non-ambigous way, so that all logical answers, not only the textbook ones, are considered correct? Or were the students actually badly prepared, or is the subject too hard for the level it was taught?
In Europe, an exam review is often fone so the students could see how their exams were graded and can argue about certain legit misunderstanding of a question (badly phrased), and check whether it was graded at all fairly. In the US, I think, it is not the case.
I hope this helps.
