Actually, if you look closer you'll see that although 'incomprehension's' did get 1 result, the entry refers to "incomprehension' without -'s or -s. Do a search on Google and see how many pages "incomprehension's" gives you.
The doctor visits the sick patient.
The doctors visit the sick patient.
The doctor's visit to the sick patient.
The doctors' visits to the sick patient.
Figure out the difference and you'll know even more than you already do!
You used it in improperly in your sentence, though. It appalls me that you failed to understand that. Maybe this reply was an easier way of coming off as less stupid, but Chronic didn't help.
Seriously, I hate it when I see educated people who make simple grammatical mistakes. I don't expect SiCc to know, he went to art school.