VOD Underlying vs SSF

[Pots]

Yesterday VOD underlying went up 1.75 but SSF only went up 65c. I understood SSF's moved by similar values to underlying price. Does anyone know why there is such a large differential betwen the two prices

[SimonPB]

probably MTM for tuesday was not great .. and you scored then an lost yesterday ..

[Pots]

Monday underylying unchanged from Fri but SSF (MTM) down by 4. Tues underlying down by 15c and ssf (MTM) down by 16c. Differential between underlying and SSF (MTM) has been within 10%-20% for most of the time so was surprised to see almost 170% difference. Maybe it will correct itself today??