The Voh and Vol specs have a range of (Vddq/2-0.12) V to (Vddq/2+0.12) V
This is defined by the QDR consortium.
Basically, for an impedance variation from 35 ohms to 70 ohms, there is a corresponding current Ioh and Iol. Corresponding to a specific Ioh and Iol current the Voh and Vol values will be different.
Hence the receiver can make out the difference between Voh and Vol for a specific impedance setting.
The DC test specifications for Voh and Vol which are specified by Notes under the DC Electrical Characteristics section of the datasheet are actually for calculating the Ioh and Iol level of the drivers for a Voh and Vol of Vddq/2. The reason +/-0.12 is added is to provide the voltage variation for the Voh and Vol based on the +/-15% accuracy of the output impedance circuitry. For this +/-15% accuracy you will see a voltage variation of +/-0.12 on the outputs.
The IOH and IOL values can be calculated as below:
|IOH|=(VDDQ/2)/(RQ/5)+/-15% for 175Ω ≤ RQ ≤ 350Ω.
|IOL|=(VDDQ/2)/(RQ/5)+/-15% for 175Ω ≤ RQ ≤ 350Ω.
For eg: for a VDDQ=1.5V and RQ=250Ω
If we consider the -15% accuracy for 50ohm you are looking at 42.5ohm output
The difference between the nominal and variation is 2.4mA
Vol variation = 0.0024*50 = 0.12V.
The same explanation holds true for Voh.