Why are Veda called apauruṣeya -अपौरुषेय , means "not of a man" ?

The Veda was called अपौरुषेय, apauruṣeya- not because no one remembers who their authors are, but because the author was, is and will be the Almighty God.

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The ancient seers, unlike later day commentators, knew their limitations and the power of Almighty.

They stated the Veda as apauruṣeya, because the contents in the Veda are divine and eternal. 

Out of humbleness, knowing fully well the divinity of eternal laws -sanAtana dharma, they called those laws as apauruṣeya.

Rig Veda III.3.1.

वैश्वानराय पर्थुपाजसे विपो रत्ना विधन्त धरुणेषु गातवे | अग्निर्हि देवानम्र्तो दुवस्यत्यथा धर्माणि सनता न दूदुषत ||

To him who shines afar, Vaiśvānara, shall bards give precious things that he may go on certain paths: For Agni the Immortal serves the Deities, and therefore never breaks their everlasting laws.

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Any Natural law is divine, whether it was/is/will be observed by a human of Eastern or Western hemisphere of our Earth.

Let us consider the law of Gravitation. 

This law was in existence even before Newton, is in existence now, and will be there after us, because it is a Natural Law, thus it is Divine.

Newton, though a great person, yet a human, claimed name for his discovery of that Law.

Ancient seers, who discovered or heard the Divine laws and transmitted them as Veda, did not claim the authorship because they were elevated souls.

That is the difference.