MNF Google Ad Cost Per Click

I have a question that is getting me thinking.

Say you have two key words, one short tail and one long tail

Weight Loss $20 per click

Weight loss techniques for overweight people $0.38 per click


Ok so you can't rank for the first kw easy cause of high soc, so you go for the second and your page hits page one. Someone then goes onto your site, ok here is the question. When on your page, your page has ranked for the long tail but that is irrelevant if it had of ranked for the short tail it would/could still have the same content on the homepage.

The ads displayed will still be about weight loss. So how does google know to attribute the second cost per click and not the first? And what if at the end of your article you have an upsell link that says to read more about this interesting topic click here then the next page on your site is just "weight loss" as the title. I bet the same ads are still displayed.

And then you have a page optimised for the short tail keyword what if someone then on that page clicks the ad. Will you recieve the higher cost per click?

Someone help me figure that enigma out