This displays the actual Refresh Rate in milliseconds i.e. 1000ms = 1 second.

The figure in brackets denotes the time in milliseconds that it took to get a response from Betfair's API server but applies to polling only.

 

When using streaming the figure in brackets denotes the difference in milliseconds between the timestamped streaming data and the time on the PC being used to receive the streamed data.

 

It's important to keep your PC clock synced in order for the figure in brackets to be accurate although it should be noted that there is a reliance on the Betfair server clock being accurate.