Cards not cracking at the same speed

I have three R9 290 in a single server and I have noticed lately that there are huge differences in the speed at which the cards are cracking passwords (see attachments).

I am running oclHashcat in Windows and I was wondering if perhaps my AMD Overdrive settings were not optimal (all cards have the same settings)?

Thanks for your help!

  p1.JPG (Size: 73.5 KB / Downloads: 2)

  p2.JPG (Size: 51.77 KB / Downloads: 2)