When I checked my Google Webmaster Tools I found that my website is showing up for keywords that I didn't optimize for ...

for example I optimize my website for "funny pictures with captions", and the website is showing up for "funny images with captions". I know that this is good, but the keyword is dancing all around,

sometimes I search for "funny pictures with captions" and I show up in the 7th page, and some time I don't show up. and the same goes for the other keyword. of course I am optimizing for more than two keywords but the results is not consistent.

my question is how does Google decide which keywords you website should show up for?

Is it the on-page keywords?, or is it the off-page anchor text keywords?

Thank you in advance ...
FarrisFahad