Im stuck on understanding how EV calculcate timers. I have this code:

#!/usr/bin/perl

use common::sense;
use AnyEvent::HTTP;
use Data::Dumper;


sub do {
    my $iter = shift;
    # It will resolve every timeout problems, but it is not question
    # AnyEvent->now_update;
    my $cv = AnyEvent->condvar; 
    $cv->begin;
    http_get
        'https://imasheep.hurrdurr.org/category/about.html',
        timeout => 2,
        sub {
            my ($body, $hdr) = @_;
            say "$iter $hdr->{Status}";
            unless ($hdr->{Status} == 200) {
                warn Data::Dumper->Dump([$hdr],['hdr']);
            }
            $cv->end;
        }
    ;
    my $timer = AnyEvent->timer(
        after => 300,
        cb    => sub { warn "Exit by timer"; $cv->send },
    );
    my ($n, $t) = (AnyEvent->now,  AnyEvent->time);
    my $d = $t-$n; 
    say "$iter diff = $d, time = $t, now = $n";
    $cv->recv;
}

for (1..4) {
    &do($_);
    sleep(5);
}
[download]

output:

1 diff = 0.0233771800994873, time = 1427912097.54858, now = 1427912097
+.5252
1 200

2 diff = 5.0014123916626, time = 1427912103.05933, now = 1427912098.05
+791
2 596
$hdr = {
         'Reason' => 'Connection timed out',
         'URL' => 'https://imasheep.hurrdurr.org/category/about.html',
         'Status' => 596
       };

3 diff = 5.00508308410645, time = 1427912108.06631, now = 1427912103.0
+6123
3 200

4 diff = 5.00244045257568, time = 1427912113.56622, now = 1427912108.5
+6378
4 596
$hdr = {
         'Status' => 596,
         'Reason' => 'Connection timed out',
         'URL' => 'https://imasheep.hurrdurr.org/category/about.html'
       };
[download]

Iteration 2 - This is OK for me. Timer was set when internal time has 5 sec lag
Iteration 3 - But I do not understand this case. Everything is the same. Why there is no timeout ???
Iteration 4 - And once again. So every even call to do() generate timeout, and every odd call doesn't generate.
Could you please clarify me about this behaviour?