The output voltage is ratiometric with the +5 volt supply (+5V),
which is generally accurate to ±5% (see Appendix A).
If an output voltage of 5 volts is specified,
the resulting output will be 100% of the supply voltage.
Similarly, specifying 2.5 volts actually gives 50% of the supply voltage.
The maximum output voltage is almost 100% of +5V at no-load, and decreases with load.
See the specifications in Appendix A relating to maximum output voltage.
Also note that loading either analog output will cause an IR drop through the source impedance of each.