everything really.
Don't they each have a buy and and a sell price and by my experience, the prices vary from the open market by a pre-determined x ticks extra spread.
e.g. S&P 500 e-mini futures. Typical spread on open market is 0.25. For the day-traded version of that at CityIndex the spread is an extra 0.1 on either side of the trade i.e. 0.45 but they round that down to 0.4
So if the market is trading Ask 1340.25 and Buy 1340.00 , CityIndex would be at 1340.3 and 1339.9 respectively. And I can say that holds consistently... to the tick.
The list at Market Information | Indices Spreads | City Index
is correct. And the prices follow the open market.
So, again I ask what data exactly are you thinking is present in their platform but would be insufficient or absent from any other?