He probably got one of the deals in the hot deals forum. I don't know about other ups's but on the APC line they work one of two ways. On the high end ones like the Back UPS Pro line they have voltage regulation. If it's too much voltage it clamps it down, if it's not enough voltage it adds power to the what the line is sending to bring it up. 
Now on their lower end series like the non-pro Back UPS's when an under or over voltage occurs it just cuts the line power and runs on battery until the conition rectifies itself. Downside is in a brownout is it's running only on battery and is only as good as the rated runtime of your battery.
So basically his $3 ups kicked off the line voltage and ran the computer on battery. 
At work all the workstations are on cheaper kickoff the line in under/over voltage, but the servers are on ups's with voltage regulation. 
APC in my opinion makes good stuff.