window function ordering not working as expected

Greetings,
I have a postgresql-9.3.x database with a table with a variety of date
stamped test results, some of which are stored in json format
(natively in the database). I'm attempting to use some window
functions to pull out specific data from the test results over a a
time window, but part of the results are not making sense. Some tests
run every day, others less frequently. For each unique test's results,
I'm interested in seeing:

* the date for the most recent result
* test name (identifier)
* most recent result (decimal value)
* the worst (lowest decimal value) test result from the past 21 days
* the date which corresponds with the worst test result from the past 21 days
* the 2nd worst (2nd lowest decimal value) test result

Here's a sample of the data and resulting score for one test (tname)
from the past few weeks:

tstamp | tname | score
------------+-----------------+--------
2015-02-17 | dfw001.ix-cr-02 | 0.7
2015-02-15 | dfw001.ix-cr-02 | 0.6
2015-02-14 | dfw001.ix-cr-02 | 0.6
2015-02-14 | dfw001.ix-cr-02 | 0.7
2015-02-13 | dfw001.ix-cr-02 | 0.6
2015-02-12 | dfw001.ix-cr-02 | 0.7
2015-02-11 | dfw001.ix-cr-02 | 0.7
2015-02-10 | dfw001.ix-cr-02 | 0.7
2015-02-09 | dfw001.ix-cr-02 | 0.7
2015-02-08 | dfw001.ix-cr-02 | 0.7
2015-02-08 | dfw001.ix-cr-02 | 0.5
2015-02-07 | dfw001.ix-cr-02 | 0.7
2015-02-07 | dfw001.ix-cr-02 | 0.5
2015-02-06 | dfw001.ix-cr-02 | 0.7
2015-02-05 | dfw001.ix-cr-02 | 0.7
2015-02-04 | dfw001.ix-cr-02 | 0.7
2015-01-30 | dfw001.ix-cr-02 | 0.7

Here's the SQL query that I'm running:

SELECT * FROM
(SELECT tstamp,
concat_ws('/',attrs->>'RCluster ID',
regexp_replace(replace(replace(attrs->>'ASN HTML','</a>',''),'<a href=
''http://ncapp100.prod.com/Cluster3.php?asn=',''),'\d+(&d=5d''
target=''_blank''>)','')) AS tname ,
metrics->>'PT TWBR' AS score,
first_value(metrics->>'PT TWBR') OVER
(PARTITION BY concat_ws('/',attrs->>'Route Cluster
ID', regexp_replace(replace(replace(attrs->>'ASN HTML','</a>',''),'<a
href= ''http://ncapp100.prod.com/Cluster3.php?asn=',''),'\d+(&d=5d''
target=''_blank''>)',''))
ORDER BY metrics->>'PT TWBR') AS worst_score,
first_value(tstamp) OVER
(PARTITION BY concat_ws('/',attrs->>'Route Cluster
ID', regexp_replace(replace(replace(attrs->>'ASN HTML','</a>',''),'<a
href= ''http://ncapp100.prod.com/Cluster3.php?asn=',''),'\d+(&d=5d''
target=''_blank''>)',''))
ORDER BY metrics->>'PT TWBR') AS worst_date,
lead(metrics->>'PT TWBR', 1) OVER
(PARTITION BY concat_ws('/',attrs->>'Route Cluster
ID', regexp_replace(replace(replace(attrs->>'ASN HTML','</a>',''),'<a
href= ''http://ncapp100.prod.com/Cluster3.php?asn=',''),'\d+(&d=5d''
target=''_blank''>)',''))
ORDER BY metrics->>'PT TWBR') AS prv_score
FROM btworks
WHERE
age(now(),tstamp) < '21 days'
ORDER BY tstamp DESC, rank
) AS stuff
WHERE
tstamp = '2015-02-17';

Here's the data from the above query as it pertains to the data
(tname='dfw001.ix-cr-02') set that I posted above:

tstamp | tname | score | worst_score | worst_date
| prv_score
------------+-----------------------+-------+-------------+------------+-----------
2015-02-17 | dfw001.ix-cr-02 | 0.7 | 0.5 | 2015-02-08 | 0.7

The problem that I'm seeing is in the prv_score column. It should show
a value of 0.6, which corresponds with 2015-02-13, however instead its
returning 0.7. I thought by ordering by metrics->>'PT TWBR' I'd always
be sorting by the scores, and as a result, the lead(metrics->>'PT
TWBR', 1) would give me the next greatest value of the score. Thus my
confusion as to why ORDER BY metrics->>'PT TWBR' isn't working as
expected.

thanks in advance for any pointers.