Smart Pricing or smart pricing is one of the security features of Google Adsense for advertisers. This system aims to reduce the risk of invalid clicks that harm the advertisers.
The workings of this system, is to set the value of ad clicks vary depending on several factors, such as: the relevance of the site, the amount of traffic, PR (page rank), aged websites / blogs, relevant links and so forth. As a result, an ad that offers a 'digital cameras' value will be higher if it appears on a site / blog that reviews about digital camera than if it appears on your site / blog about photography for example.
On the Support page of Google itself mentioned:
"Smart pricing is a Google feature that may reduce your cost-per-click (CPC) on the Google Network.
We are constantly analyzing data across our network against a variety of factors. If our data shows that a click from a Google Network page is less likely to turn into actionable business results - such as online sales, registrations, phone calls, or newsletter registration - we may reduce the bid for that page.
We consider many factors such as keyword lists or concepts which triggered the ad as well as the type of page where the ad is shown. For example, you advertise digital cameras. Your ad appears on two different pages - a Google web search page for the keyword 'Digita cameras' and a Google Network page about digital cameras. If Google determines that your ad will likely not perform as well on the Google Network page as on Google web search, the AdWords system may reduce the maximum CPC bid for that site.
Google saves you time and hassle by estimating the value of clicks and adjusting bids on an ongoing basis. Without any extra effort from you, Google technology helps you get strong ROI from your AdWords ads. "
From the above, one of the most important one is, the relevance of the topic of the blog. This is the main reason why should we only manage one topic for your blog. In addition to managing only one topic in blogging, we are more able to get relevant ads.