Hi,
Your list of sample times range from 3376 to 3384, that's only +-4 millionths of a second.
Also notice that the deviations come in pairs:
6699068 3380
6702448 3384 +4
6705828 3376 - 4
6709208 3380
The resolution of the micros() probably only counts in 4's so you occasionally see plus or minus 1 count.
Don't worry about it.
Yours,
TonyWilk