As for the original post, you actually have two auto transformers, as a Line Interactive UPS has an auto transformer internally.
I'd assume one of them is limiting the current flow.
However as it only takes 20mA to stop the heart, even low currents can be lethal.
To be honest a full isolation transformer would be better, as far as limiting the smoke output of whatever failed device you have connected to the output.
Sorry, I only have one that inside the UPS, I don't have another one..