After setting cpu affinity using taskset, why does 'ps' still show process running on cpu outside new affinity?

Issue

• After setting cpu affinity using taskset, why does 'ps' still show process running on cpu outside new affinity?

# ps -eo pid,psr,comm|grep cat
 2635   3 cat

# taskset -cp 1 2635
pid 2635's current affinity list: 0,3
pid 2635's new affinity list: 1

But 'ps' still shows cpu#3

# ps -eo pid,psr,comm|grep cat
 2635   3 cat