[daveFNbuck]

That professor probably had produced the answer already, using the metric he had initially intended. Having a set of answers doesn't mean you didn't make a mistake in the questions.

[saagarjha]

But it does clue you into whether the professor used a different set of assumptions than you did.

[daveFNbuck]

Presumably you wouldn't see the answers until after you spent all night on the assignment.