Ahhhh i found the source of the confusion, once you start getting into power the units start getting confusing.
V and VA are not the same thing, VA(Volt-amps) is a unit of power with a slightly different definition from watts. Simple loads like toasters draw the same number of watts and volt-amps, more complicated loads like a motor with its big inductive coil will draw more volt-amps than watts, but active PFC makes computers look more like toasters so we will work off of watts for the sake of this discussion.
The UPS will output the same voltage as it takes in from the wall, lets say 100VAC(Volts-alternating current) for easier math, if the UPS has a 600VA capacity and a 600W capacity(easier discussion) then the computer can draw up to 6A from it, this will be a total draw of 100V*6A=600VA=600W.
Now if the UPS doesn't have a large inverter to it but is hooked to a massive battery bank it might have a 1200Wh(watt-hour) battery bank, this means that you can pull 200W for 6 hours, or 600W for 2 hours, or 1200W for 1 hour from that battery bank, but you can't pull 1200W from your 600W inverter down stream so if your load is trying to pull 1200W even though the battery bank could support that for an hour the inverter that converts it from DC-AC voltage cannot so the inverter will shut off to protect itself turning off power to the PC.
For UPS's because they are batteries feeding an inverter there are two primary stats, what is the maximum output of the inverter(Watts is the figure important for computers), and what is the hold up time for a specific load(this is determined by the size of the battery bank)