R9 Fury Nitro 1100 mhz gpu clock - so slow - why ?

I've tested a R9 Fury Nitro with a to 1100 mhz overclocked gpu. 

It needs for 8 digits wpa2 crack 3 hours. A GTX 1080 needs under 10 minutes.

GTX 1080 = ~400 kH/s

Fury Nitro = ~233 KH/s

So the R9 Fury Nitro  should need ~ 15 minutes.

The reason for this astonishing result of 3 hours is maybe the the amount of data. 

Hashcat shows:

2.500.000.000 hashes to test

8 digits (0 - 9):

10^8 = 100.000.000 hashes 

So hashcat tests 25 times more than it should be.

Any reason for this bug ?

Used hashcat version is 3.30.

Best regards

Millenium251