Because with ordinary iron sights there are two, one at the back of the barrel, one at the front, and you align both of those with your target.
They can be made to not be quite 100% parallel with the barrel, but instead fractionally downwards.
But all that aside, they’re a guide only. A set of iron sights is only good for a specific distance, beyond or within that, you’d have to make judgements based on experience to make an accurate shot anyway.
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