I dispute that this question is really a duplicate:
The duplicate question to which the asker is referred explains the Great Vowel Shift, but the OP is not asking about the GVS. He's asking, instead, why two different words with the same orthography have different pronunciations. The answer to his question may involve the GVS, but a real answer to his question should include some explanation of the words' etymologies, whether they were once spelled differently, etc.
I bring this up on meta rather than simply commenting, because this is not the only question I've seen closed in this matter. Not all questions about the spelling of English vowels are duplicates of a GVS question. In particular, questions about the differences in pronunciation between two words can never be completely answered just by an explanation the GVS.