Because in theory the console gpus were supposed to perform twice as much as the equivalent PC gpus and that is very clearly not the case.
It's not as clear cut as you think. The unified architecture allows for possibilities you don't have on PC. I could design an algorithm, which runs a lot faster on PS4 than a PC with 3 of the newest Titans, although the PS4 could never beat the Titans in rendering performance:
Example:
Imagine your CPU has an instruction set for encrypting data in hardware and you also wrote a very fast parallel algorithm to generate the data on GPU.
On your PC this algorithm looks like this:
GPU: Generate Data, 2ms
Send Data to CPU, 5ms <- no matter the GPU or CPU, you will not get this step significantly faster
CPU: Encrypt Data, 2ms
On PS4 it looks like this:
GPU: Generate Data (slower): 4ms
CPU: Encrypt Data (slower): 4ms
Even though the PS4 in my thought experiment is twice as slow as the PC, it executes the algorithm 1ms faster, because it eliminates the intermediate step. The only thing I want to clear up here is: Yes, the PS4 is globally slower and will execute any PC optimized algorithm with the same speed as an equivalent specced PC, but it also allows to execute algorithms very fast, which would have horrendous runtimes on PC.
Disclaimer: I have absolutely no proof for the following statement, it's just a theory.
One possibility explaining Arkham Knight performing so badly in the beginning on PC is that many effects used the GPU for computing on Console without accounting for the fact that this induces latency on PC and doesn't on console.