Hmm this link gives a basic overview of why - http://www.maxmax.com/auvgermicidaltechnicaldata.htm
Mostly the kill rate of ultraviolet depends on the intensity of the bulb and the microwattage per second required to kill a specific bacteria. If you have a UV bulb that only produces say 700 microwatts which would be fairly close to the output of a 5W bulb then you need to divide the wattage to kill algae which is 22,000 by that number to get the amount of seconds you would need to expose the algae to the light to kill it.
As you can see it would take a 5W UV light a bit over thirty one seconds to kill an algae cell.
The lifeguard aquatics one I'm looking at is 15W and produces 12,470 microwatts so it would only take a bit over two seconds.
The problem is its difficult to find a site that will actually put out the microwatt results of their equipment and the only sites to do so also produce commercial sewage waste sterilizers and are required to prove they meet EPA required guidelines for that usage.
BTW the 700 guess is based off that site but really alot depends on the quality of the light and the quartz sleeve around it.