As we know that, No order guaranteed if order by is notf  explcitly specified. Below is one more example of this.

Below is the code to pupolate dummy table.

If OBJECT_ID('dbo.testTable') is not null 
	drop table dbo.testTable
Go
Create table dbo.testTable
( c1 int not null,
c2 int not null,
c3 int not null,
c4 int not null)
go
Alter table dbo.testTable add constraint pk_testTable primary key (c1)
go
declare @i int
select @i = 1

set nocount on;
begin tran

while (@i < 1001)
begin
	insert into testTable (c1,c2,c3,c4)
	select @i, @i/2, 10000-(@i/4), @i%2

	select @i = @i + 1
end
commit
go

Now, when we run below query, have a look at rows returned

select * from testTable

Below is the truncated result of this query.

Now, if we create covering below index and again look at the results, rows are returned in different orders.

Create index idx_testTable_c3 on testTable(c3) include (c1,c2,c4) 
go 
select * from testTable

Below is the result returned after creating index

Actually, SQL choose the plan which is the more efficient, when we run below queries, we are able to find the reason.

set statistics io on 
go 
select * from testTable with(index=0) 
go 
select * from testTable go

Even we can look into why there are less pages when nonclustered index is used, with help of below query

select * from sys.dm_db_index_physical_stats(db_id(),object_id('testTable'),NULL,NULL,'DETAILED')