Why are not all SMs active when NCCL kernel and compute kernel overlap?

[yu-depend]

When I run a single NCCL kernel ,the active SMs is 15%，and When I run a single compute kernel ,the active SMs is 100% ，but when I run the compute kernel and the NCCL kernel in parallel, so that they overlap，the active SMs is 85%, how to explain this？