For the 25v tolerant inputs the voltage is divided by a basic resistor divider which scales 25.85V down to 3.3v.
I go into a little more detail about what this means for sensors with a low-voltage output here: Automation phat compatible sensors
On Automation HATs 3v3 header there is a 4th, unscaled, 0-3.3v ADC input which you could use.