You seem to miss something in your logic.
If a player scores more out of more chances, he may not be as lethal, but he's producing better.
As you said "their job is score goals". Cavani does it better than anyone, not because he's lethal but because he masters the art of being at the right place at the right time and ALWAYS offers his teammates a passing chance.
There are some players out there who are lethal but have low IQ and are never in a position to score. That's of course not Aguero or Kane's profile, but Cavani is much better at this than both.
No way you take Cavani over Kane. To each his own. I know I'd go with Cavani for a war on a pitch. He's the ultimate warrior. He'll do everything to win. He'll run for 90 minutes and if I have good passers, he's my choice. Because he'll get 5 golden chances and score 2. He might bottle the other 3, but other strikers wouldn't get more than 2 golden chances and score 1.