iRacing Relative gaps don't match SimHub data

Morning. I am trying to dial in a vocore screen iRacing relative with SimHub and the time gaps do not match iRacing's relative box. The gap to the car behind is within a few tenths of the iRacing F3 box but the gap ahead is, up to, three seconds out on the vocore screen. 

Has anyone else experienced this or have any idea how the data can be so different to iRacing please?