I2 Lap Varience & Lap TIme Difference

I have an SDL3 with pro logging. I have noticed that the variance and lap times between 2 laps do not match. If you look at the attached log the laptime difference for lap 5 and 7 is 2.238 secs but the variance is 2.6secs. Is this a common problem ? Possible sources of the difference or am I expecting too much ?

Brian,

What was the outcome of the suggestions made last time you asked about this?

https://www.hpacademy.com/forum/data-analysis-fundamentals/show/motec-varience-doesnt-match-lap-time-difference

And for now, what rate are you logging all the related items at? I often see people log lap time related items at very low frequencies, so this came to mind.