Of course a test like this does not compensate for all variables. Every piece of in the chain is going to have an effect, from mouse, to your specific computer hardware, operating system, all the way to your display output.

That said, I think your delta of 30ms was just variance that would be reduced by averaging several results. The maximum added latency by using 60hz is 16.66ms, while the maximum average for 75hz is 13.33ms, a far cry from the 30ms you experienced.