Reason alias works in ORDER BY but not in WHERE

Question

I understand that I can't reference an alias in the WHERE clause, but why is that? Is it interpreted differently?

Something like this generates an error:

declare @myTable table 
(
    num numeric(5,2),
    den numeric(5,2)
)
insert into @mytable 
select 1, 2
union
select 1, 3
union
select 2, 3
union
select 2, 4
union
select 2, 5
union
select null, 1

select num/den as 'calc' from @myTable
where calc is not null
order by calc

But this returns rows:

declare @myTable table 
(
    num numeric(5,2),
    den numeric(5,2)
)
insert into @mytable 
select 1, 2
union
select 1, 3
union
select 2, 3
union
select 2, 4
union
select 2, 5
union
select null, 1

select num/den as 'calc' from @myTable
--where calc is not null
order by calc

Answer 1

As mentioned in Cannot use Alias name in WHERE clause but can in ORDER BY, it's due to the natural query processing order:

1. FROM
2. ON
3. OUTER
4. WHERE
5. GROUP BY
6. CUBE | ROLLUP
7. HAVING
8. SELECT
9. DISTINCT
10. ORDER BY
11. TOP