Why is this 5V voltage regulator not outputting 5V?
Two things:


It sounds like you aren't using an output capacitor. Note the schematic on page 4 in the datasheet. "10 µF is the minimum value required for stability and may be increased without bound." Without this capacitor, the output is likely oscillating, which your meter might read as a funny voltage.
It sounds like you have no load attached to the output. Load regulation is only specified for loads between 5 and 150 mA. Below 5 mA, the output voltage is likely to increase, at least slightly.