One RX470 4GB runs slower than the rest

[chiwalfrm]

Mining ETH on Ubuntu 16.04.3 LTS. I noticed one of the four GPU's hashes much slower than the other three.
Doing the following command (as root) shows why.

# cat /sys/kernel/debug/dri/*/amdgpu_pm_info | grep -i sclk | cat -n
                         1          898 MHz (SCLK)
                         2          1130 MHz (SCLK)
                         3          1130 MHz (SCLK)
                         4          1130 MHz (SCLK)       

The core clock is running at only 898 MHz vs the normal 1130. Now I check memory clock.

# cat /sys/kernel/debug/dri/*/amdgpu_pm_info | grep -i mclk | cat -n
     1          2000 MHz (MCLK)
     2          2000 MHz (MCLK)
     3          2000 MHz (MCLK)
     4          2000 MHz (MCLK)

And everything is as expected at 2000 MHz.

All GPU's are identical RX470 4GB running the exact same BIOS. Why would identical cards with identical BIOS have one odd one with slow core clock?
Temperatures are all normal for all four at 63 degrees C.