Why doesn't BOINC use the CPU power fully on my Huawei P9 lite?
Yes it is true, the Huawei P9 lite has a Octa core CPU, however not all 8 cores are the same. Instead it follows the ARM BIG-little concept:

The 4 "BIG" cores (2.0GHz) are dedicated for maximum CPU speed. They are optimized for maximum performance (at cost of power consumption). 

The 4 "little" cores (1.7GHz) are optimized for minimum power consumption. They are pretty slow compared to the BIG cpu cores (more than the 15% the cpu speed comparison indicates) as they are mostly used when the phone is in stand-by mode and does not have to compute much. 

Therefore it is not unusual that only 4 of 8 cores are used. You should use a system info app that displays the current (not the maximum) CPU clock speed of all 8 cores. You should see that 4 cores work at 2.0 GHz - so the BIG cores are used.
For my device the app CPU-Z works and displays the clock speed (the refresh speed is not very high so you have to wait some seconds).