Iranian Persians, please come here?

Why do you say barun when it is spelled باران? why not baran?
Why do you say khune when it is spelled خانه? why not khane?
Why do you say iruni when it is spelled ایرانی?

There are many other words where one of the ا is pronounced as u. I hear it a lot when Iranians talk and also in songs. Why is it like this?
Update: But WHY is it like this? It makes no sense to me?
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