i have 13 GTX 1060. Why GPU 1 --> 6 ~ 9k Sol/s but  GPU 7 --> 13 ~ 7k Sol/s. Please help me T_T

[Device 0] Average speed (5s): 9382.12 sol/s
[Device 1] Average speed (5s): 9522.9 sol/s
[Device 2] Average speed (5s): 9543.41 sol/s
[Device 3] Average speed (5s): 9518.32 sol/s
[Device 4] Average speed (5s): 9527.48 sol/s
[Device 5] Average speed (5s): 9473.32 sol/s
[Device 6] Average speed (5s): 7309.84 sol/s
[Device 7] Average speed (5s): 7378.34 sol/s
[Device 8] Average speed (5s): 7459.38 sol/s
[Device 9] Average speed (5s): 7189.37 sol/s
[Device 10] Average speed (5s): 7461.77 sol/s
[Device 11] Average speed (5s): 7315.41 sol/s
[Device 12] Average speed (5s): 7324.77 sol/s