If a new user close too fast, the "Page views" will not be captured but “Unique Visitors” will be captured?

Dear All:

I found that in my reports there are less page views than unique visitors. 307 Unique Visitors but only 300 page view? If a user visits Portal at least one "Page views" is counted，so "Page views" should be greater than or equal to the unique visitor, why the count of Page views less than Unique Visitors .

If a new user close too fast (how many seconds?), the "Page views" will not be captured but “Unique Visitors” will be captured?