Then they have already subtracted their margin of error, which is definitely above 2km/h.
More likely they saw e.g 66km/h in a speed gun with 4km/h error, so they know you did at least 62, and can book you stating 62. But most likely you were doing 66 or even 70 then.
In NZ, which generally copy-pastes VIC's traffic laws, the tolerance is only applied to automated systems like speed cameras. A cop can ticket you in-person when you're 1km/h over.
