why nr_involuntary_switches of a process runs on an isolated cpu is constantly increasing?

I have a server (linux 4.18+intel xeon) which has isolated cpus for latency-sensitive service. I noticed that the service process's nr_involuntary_switches is gradually increasing every a few seconds by looking at the /proc/{pid}/sched. My questions is: since the cpu is dedicated for the service process, how come there is still context-switches?