COUNT(*) over windowing 'x preceding and y preceding' doesn't work properly

select ts, f, count(*) over (partition by ts order by f rows between 2 preceding and 1 preceding) from over10k limit 100;

2013-03-01 09:11:58.70307       3.17    0
2013-03-01 09:11:58.70307       10.89   0
2013-03-01 09:11:58.70307       14.54   1
2013-03-01 09:11:58.70307       14.78   1
2013-03-01 09:11:58.70307       17.85   1
2013-03-01 09:11:58.70307       20.61   1
2013-03-01 09:11:58.70307       28.69   1
2013-03-01 09:11:58.70307       29.22   1
2013-03-01 09:11:58.70307       31.17   1
2013-03-01 09:11:58.70307       38.35   1
2013-03-01 09:11:58.70307       38.61   1