Database Table Primary Key Question For Database Experts

Hi, 
I have a Users table that I use for membership. Now I am using username varchar(30) as the primary key for this table since username will always be unique.
I am storing username in UsersInRole table so that I can find what roles each user has.
The question I have is regarding how SQL Server actually stores data:
I see that when I add users, they are always stored alphabetically sorted on username.
I was expecting that all users will appear on the users table in the order they were added.
Example: I have 3 users (john, jonah, wilson). Now I added 4 user with username='bob'
If I execute select * from users, it returns me (bob, john, jonah, wilson). Look bob is has become the first row of the table.

My question: Is Sql server moving 3 older rows to make room for 'bob' and it is also rebuilding part of the index due this new username 'bob'?

If this is the case, then it will have big impact if I have 100K users and I add one user that becomes firstrow. In that case huge other rows will have to move.

Bottom line, insert, delete will be expensive.
I know sql server keeps data physically sorted on PK. But I am concerned here since rows are losing the order in which they were inserted.
Thanks