There is also the possibility that you have chosen a more expensive instance class and those are converted to B1 hours. The instance hour is $0.05 for instance class B1 (the lowest class). If you upgrade to higher classes (faster CPU and/or more RAM), the instance hours are more expensive.
https://cloud.google.com/appengine/docs/python/modules/#Python_Instance_scaling_and_class
In the reports however, all instance hours are converted to instance hours of B1. So if your app consumed 1 hour of B4 instance class, the report and the billing will show it as 4 instance hours (B1), because B4 costs 4 times of B1 ($0.20 vs. $0.05).