Column aliases in GROUP BY and HAVING

In my mental model of how SQL works, the clauses of an SQL query (if
present) are processed in a certain order:

FROM
WHERE
SELECT
GROUP BY
HAVING
ORDER BY
LIMIT

and each processes the output of the previous one.

However, consider this:

hjp=> select * from employees;
╔════════════╤═══════╤════════════╗
║ ssn │ name │ other_data ║
╟────────────┼───────┼────────────╢
║ 1234010400 │ Alice │ (∅) ║
║ 2345180976 │ Bob │ (∅) ║
║ 2645101276 │ Carol │ (∅) ║
║ 9843100395 │ David │ (∅) ║
╚════════════╧═══════╧════════════╝

hjp=> select substring(ssn, 9, 2) as year, count(*) from employees group by year;
╔══════╤═══════╗
║ year │ count ║
╟──────┼───────╢
║ 95 │ 1 ║
║ 76 │ 2 ║
║ 00 │ 1 ║
╚══════╧═══════╝
(3 rows)

In the GROUP BY clause I can use the alias year which was defined
earlier in SELECT.

HAVING comes after GROUP BY, so I should be able to use that there, too.
Right?

hjp=> select substring(ssn, 9, 2) as year, count(*) from employees group by year having year > '20';
ERROR: column "year" does not exist
LINE 1: ...ear, count(*) from employees group by year having year > '20...

Wrong. I have to use the whole expression again:

hjp=> select substring(ssn, 9, 2) as year, count(*) from employees group by year having substring(ssn, 9, 2) > '20';
╔══════╤═══════╗
║ year │ count ║
╟──────┼───────╢
║ 95 │ 1 ║
║ 76 │ 2 ║
╚══════╧═══════╝
(2 rows)

This seems inconsistent to me. Is there a technical or semantic reason
for this or is just "because the standard says so".

hp

PS: Please no discussions about the appropriateness of using an SSN as
an id. This is a completely made-up example.

--
_ | Peter J. Holzer | Story must make more sense than reality.
|_|_) | |
| | | hjp(at)hjp(dot)at | -- Charles Stross, "Creative writing
__/ | http://www.hjp.at/ | challenge!"