We've only got 4 chapters to go in Greek 1 and am thrilled at the progress of the class. But the more you study a language, the more exceptions to the rules you also have to learn. This goes for English as well. See these words?
They all end in -ough but are pronounced completely differently! In English, we have "to," "too," and "two," but in German these become zu, auch, and zwei. Of course it makes no sense. I could go on and on.
The bottom line? When learning a language, there will always be exceptions to the rules. All languages have "That's just the way it is" characteristics!