How are you powering it? You should get a minimum of 4v if power is 5v, according to the datasheet. That means usb power or 7v minimum on Vin. 5 volts on Vin could cause that.
An unloaded output pin will go right to the supply voltage if driven HIGH, and right down to ground if driven
LOW. Only if you load the pin and source/sink current from it will the voltage differ from these limits.