Not able find user in userinformation list

I am using one list and it has one person field configured for "All Users".

So when i type some email ID in that person field, it is getting user and it works well.

But when i wants to add new record, some users are not available in UserinformationList in c# and Javascript as well.

Here is the code which i am trying to get all users list.

 ClientContext clientContext1 = new ClientContext(SiteUrl);
 clientContext1.AuthenticationMode = ClientAuthenticationMode.Default;
 UserCollection usrdd = clientContext1.Web.SiteUsers;
 clientContext1.ExecuteQuery();