Why does the UPS mode of an inverter have faster switching times?
I would agree. It sounds like the inverter is running all the time when in UPS mode &amp; either handling the entire system load or switching the load from mains to inverter output on power failure. 

Transformer already energized = less transfer time

The increased minimum input voltage in UPS mode suggests the primary AC-DC is having to both charge batteries &amp; supply power to generate AC.

Due to inverter conversion losses you would need more power input overall to run in "UPS" mode, hence more current at the lower voltage rating. possibly exceeding some limiting rating... (speculation here)

I am unsure why the maximum input V would lower.

All of that is assuming some hybrid topology between a double conversion on-line UPS &amp; standby UPS... Double conversion offline perhaps?    

Topology info taken from:

https://download.schneider-electric.com/files?p_File_Name=SADE-5TNM3Y_R7_EN.pdf&amp;p_Doc_Ref=SPD_SADE-5TNM3Y_EN