Why are the IG Index bid/ask spreads different to those quoted on other trading sites?

Why are the IG Index bid/ask spreads different to those quoted on other trading sites?

The spreads are wider for spread betting because there are no commissions charged on the trade.

A typical IG Index spread for a daily bet will be between 6-10 points Bid/Ask.

But, in a volatile scenario, such as the market open as the pent up orders from the closed period flow through, the Bid/Ask spread at the market increases. 

This can be reflected exponentially on the spread bet.