Phoenix Historic Images page and I've been trying to puzzle it out. Google+ is being invented as we use it, and it changes regularly, so it's like watching some phenomenon happening under a microscope, you just have to try to draw intelligent conclusions. Even the Google+ people don't really know what's going on. If you read the forums you often see the experts saying "I'da know!"
The easy number to understand is followers. That's the total number of people who have your Google+ page in their stream. Just like Facebook, you really don't know if people clicked "follow" and never returned, but it's nice to at least see hundreds of followers instead of, uh, 12, which is kind'a sad. When that number goes up, it means that a new person with a Google+ account has clicked "follow" for your page. When it goes down, it can mean that they "unfollowed" your page, or they have left Google+.
Plusses, which are only visible to you, indicate the number of times a unique individual has clicked a plus for a post on your page. Once they have plussed something on your page, no matter how many other plusses they do, the plus count does not go up. Keep in mind that people can click plus on your page even if they are not following, assuming you allow it (and I can't imagine why you would not allow it). When the number for the plusses goes down, it can mean that someone who originally plussed something went back and unplussed it (not very likely) or they just got rid of their Google+ account (much more likely).
Of course, that's just the logic as of right now. Google reserves the right to change how it counts things any time it wants. It's kind of like watching a football game when suddenly the number of points for a touchdown changes. But, it's just a game.