I know it is not only the number of followers.
But it is somehow related with +1s on page's posts, page's followers and page's +1s.
Also sometimes I note that the count decreases, not sure if it is related to the bug mentioned in this article.
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Because the answer by @Fogest was not acceptable for me, I tried to post an answer myself, but still I would like to get a more accurate answer with official references.
Google mentions here that any +1's on your ads will increment the count on your page and vice versa.
It seems +1's of the Google+ page is similar to the count of +1 button on the linked page/homepage. any +1 on the page also counts as a +1 for the linked homepage.
Google posted this on their blog:
We’ll now show an aggregated count of users that have engaged with your page, either by +1’ing it or by adding it to a circle. This way, both you and your page’s visitors can get an at-a-glance summary of who is interacting with your page.
According to the above, the +1 count should include:
Also according to my personal experience the +1's on the posts of the Google+ page also adds to the to Google+ page's +1's, but I couldn't find any official reference for that.
It is the number of people who have
+1'ed the page or website associated.