Why The Nou Camp is Never Full?

Can somebody tell me why apparently the Nou Camp is never completely full? Apparently there is a single seat in the stadium which has remained empty.

This is for a quiz I am currently taking part in.

Thanks in advance for your help.

toffee

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I'm not 100% sure but i think its a safety issue. But the average attendance is well past 75,000!!!!!!!

Maybe its for respect with someone... I thought it was usually full for champions league games or ones against Real Madrid.