%Z isn't an efficiency rating and it isn't a fixed resistance value. It's the single biggest lever on how much fault current a transformer lets through.
Every distribution transformer nameplate carries a percent impedance value — %Z, typically somewhere between 2% and 10%. It's easy to glance past it as a minor spec, or worse, to assume it says something about how efficient the transformer is. It doesn't. %Z is a design property that describes how much the transformer's own internal impedance drops its voltage at full-rated current — and it turns out to be the dominant factor controlling how much fault current is available on its secondary the instant a short circuit occurs downstream.