Currently in the v15 Fall Out report, if you go from a traffic report with Page Views selected, the Fall Out value select dialog will find PV calls and the subsequent report will produce a PV based fall out report.
if you go from a traffic report with Instances selected, the Fall Out value select dialog will find Instance calls and the subsequent report will produce an Instance based fall out report.
This is very good, as sometimes we need to trend PV calls and sometimes we need to trend Instance calls. (Yes, its a feature not a bug).
The only down side is that once within the Fall Out report, there is no indication of what metric is selected and no ability to change the metric.