Nope...nothing to do with battery or charging current etc. Its the power factor. Seems like a definition is in order:
The power factor of an AC electric power system is defined as the ratio of the real power to the apparent power, and is a number between 0 and 1 (frequently expressed as a percentage, e.g. 0.5 pf = 50% pf).
Real power is the capacity of the circuit for performing work in a particular time. (This is what we measure in watts)
Apparent power is the product of the current and voltage of the circuit. (This is the VA or Volts x Amperes)
Due to energy stored in the load and returned to the source, or due to a non-linear load that distorts the wave shape of the current drawn from the source, the apparent power can be greater than the real power. Low-power-factor loads increase losses in a power distribution system and result in increased energy costs.
Typical examples of non-linear loads are fans and flourescent lights with ballast.
Now in case of UPS's (and Generators), the better the design/type of UPS, the higher the power factor. In plain english, the higher grade UPS will be able to provide "real" power almost as much as its rated "apparent" power....that is....the ratio of real power to apparent power (power factor) nearest to 1.
So you need to take note of rated Watts when purchasing UPS/Generators. Don't decide on the VA or KVA rating alone as it does not indicate "real" power the UPS/Genset can provide.