idolw - 3:01 pm on Jul 27, 2010 (gmt 0)

It depends on the audience, banner content and the landing page.
In case you pay a lot with and target the entire dsplay network your ads may show up on many sites that are not very tightly related to your niche.
A very attractive banner or one full of great promises that are not present on landing pages may be another reason.
Poor quality landing page may be another reason for failure.

I am unable to tell "typical" numbers but I have seen 60-70% bounce rates for display advertising.