Please - a bit of patience. This issue is important to you,
we understand that, but please give us a bit more than a
weekend to respond.

The online handler has a garbage collection routine so that
it doesn't check and discard timed-out online users on
every
request. Therefore it can be somewhat unprecise.

You can remedy some of that by removing the garbage
collection randomiser so it will always run.
This is done by editing
modules/system/blocks/system_blocks.php, the
b_system_online_show() function

Find these lines:
mt_srand((double)microtime()*1000000);
// set gc probabillity to 10% for now..
if (mt_rand(1, 100) < 11) {
$online_handler->gc(300);
}

and replace them with just
$online_handler->gc(300);

But I wouldn't recommend making business solutions based
on
the Who's Online block. Instead, I would suggest using a
stats tracking tool like PHP-Stats, AWStats or Webalizer,
when it is so crucial to your business.