Bing announced a new change to its Webmaster Tools, related to how it calculates Change in the Site Activity widget.
Vincent Wehren explains on Bing's Webmaster Center blog, "For the metrics Clicks from search, Appeared in search, Pages crawled, and Crawl errors we will now show you the percent change for the selected date range (as selected in the date selector near the top of the dashboard) compared to the prior date range of the same length. So when you log into your Bing Webmaster Tools account and are looking at changes for say a 30-day period in your Dashboard (which is the default setting), the percentage in the Site Activity widget will reflect your relative increase or decrease compared to the prior 30-day period."
Bing Webmaster Tools will also show you the absolute numbers for the aforementioned metrics for both the current and prior period:
"It’s worth noting that since we store up to 6 months’ worth of data in Bing Webmaster Tools, we are — as a consequence — able to show % change information for periods up to three months counting back from the last available date," says Wehren. "For shorter date ranges we can show change information as long as they and their prior period fall within those 6 months of data we store. In other words, you are not limited to just seeing change information for say, just the last 30 days."
Bing Webmaster Tools will only provide data for your site from the day your site was registered, of course.
BIng notes that the changes to the Site Activity widget also apply to change info shown on the My Sites page.