Google doesn't offer a formula for how they come up with your earnings. It is very complicated because Google AdWords is rather complicated. Those who are paying for the ads bid based on keywords. Some of those bids are really low so that Google makes a couple of pennies per click while others are highly competitive and the clicks are worth dollars. As someone who is using AdSense you get paid a portion of what Google makes per valid click that is done on your ad. As someone using HubPages your ads are shown 60% of the time. If someone comes to your hub and clicks on an ad while your ad is being shown you will make money.
Over time you will see your earnings balance out as long as you aren't adding new material. If you are adding more hubs (or other pages somewhere else) then you will make more and more money, slowly but surely.