How do Apple's USB Power Adapters allow for more power output than is possible in the USB 2.0 specification? From my understanding, the USB 2.0 specification allows for a maximum power draw of 2.5w - but Apple's own USB Power Adapters allow for a maximum of 12w using a USB to Lightning cable. How is this possible over a USB port if this power usage isn't defined as part of the specification?
In the official website, it say "USB Power Adapter" without specifying which version of the USB port, why do you think it's 2.0 version ? 

Also, the power allowance from USB 2.0 specification is for powering connected devices (mice, keyboards...). For charging, there is Battery Charging Specification

You can read more in here:
https://www.maximintegrated.com/en/design/technical-documents/tutorials/4/4803.html