The pull-downs has a weak 100 k Ohm pull-down pin, while the series resistor pins have a 1 k Ohm series resistor and a 1.37 k Ohm pull-down resistor. This will be likely be changed to a voltage divider in newer versions.
Absolute maximal ratings
The voltage on any 3.3 V pin should be within -0.3 V and (VDD + 0.3 V) with respect to VSS, where VDD should be 3.3 V.
The voltage on any 5 V pin should be within -0.3 V and (5.5 V) with respect to VSS.
Input low voltage is registered as lower than: 0.2 * VDD = 0.66 V for I/O pins.
Input high voltage is registered as higher than: 0.8 * VDD = 2.64 V for I/O pins.