Hi Eric. You are looking at two totally different scenarios. You are measuring on an input, without any external connections (except your high-impedance meter) and yet you want to use the line to drive a reset line, which is an output. Spot the contradiction?
Have a read of the datasheet, under "Electrical Characteristics", sub-section "I/O Pin Characteristics". You may also want to read up on hardware implementations of IC I/O to see how exactly they manage to achieve all the S/W-selectable variations.
The silicon hardware is different for an input vs. output. To make the input work as expected, you must have an external connection to match the logic level you are trying to read. With whatever pull-up or pull-down resistance you have selected (which the datasheet says is anything between 20-60kE) the device will read a logic low on an unconnected pin.
When you drive the pin as an output logic low, however, the FET is driven full on, giving you an output level that is maximum 0.2*Vdd.
**edited to correct error - said output when I meant input!