Almost certainly due to voltage fluctuations, especially since your roommate is experiencing the same thing.
The PSU of your computer cannot work properly below a certain voltage level. The critical level is not exactly the same for different models. Some models can work down to 180V, some to 160V. Because of this, the UPS is designed in such a way that, if the mains voltage is too low, it will treat it as a power cut and start using battery power. Hence the beeping.
The difference in the beep starting time is probably because the two UPSes have different sensitivities to changes in mains voltage levels.
If it doesn't happen often, accept it for what it is. When the beeping occurs, know that you're using battery power and start shutting yoiur computer down. If it happens regularly, use a voltage stabilizer between the mains and the UPS, not between the UPS and the computer.