I think this is just a simple answer.
Your advertisement text/title/description might not be same as your organic text/title/description. Unless they are the exact same, the bounce rate will be a bit different depending on the landing page.
I would test out the organic search's text/title/description, and create NEW advertisement(so it won't ruin your current AD), and use that organic TERM within the PPC.
Then you can check your bounce rate and see if it really differs that much.
This is the reason why many say meta data is not dead just not relevant in ranking. Meta data is still crazily important to attract the user and gives the user an idea of what to expect when clicking on the result.