Hi @keerthikoru ,

Because a role can contain several roles.

Hello @keerthikoru ,

The roles other than you added are the child role which are inherited by the parent role (which you add),

If my response helped marked as helpful and accept the solution..

Hi @keerthikoru,

It adds multiple roles because of Role Inheritance in ServiceNow.

When you assign one role, it may contain other child roles, so the system automatically adds them too.

Check:
User Role → Open Role → “Contains Roles” related list

It’s normal behavior, not an issue.

If my response helped, please mark it as the accepted solution so others can benefit as well. 

One role may inherit other roles, Like parent child, so when you get the parent Role you get the child one too.

A role can contain multiple roles. e.g ITIL role. If you check itil role 's  'contains role' tab/related list, you can see multiple roles are listed there.