The charting methods are different for charting average response time for a test (single line) and charting pages in Backbone and Last Mile tests.
A standard response time chart shows the average time for the entire test. Expanding this data to chart by page displays a response time value for each page individually.
The standard chart displays single-line, average response time only for tests that succeeded all the way through. When charting by page, if a page was successful yet the overall test run failed, the response times for that page is included. In other words, the page chart includes data that is excluded from the test chart.
For example, if you chart 10 tests but only 5 are successful, the standard response time chart is based on 5 successful tests. The chart by page information includes response time data for successful pages for all 10 tests, even if there are errors.