The period is 16.35 +/-0.18 days [1] so it's not exactly 16 days. The unit days is just the next best convenient unit to use for this range of time scale. 392 hours +/- 4 hours just isn't as intuitive as 16 days.
So the period is neither an integer multiple of days, nor exactly the same each time. There's some small variance involved - about 1%.
So the period is neither an integer multiple of days, nor exactly the same each time. There's some small variance involved - about 1%.
[1] https://en.wikipedia.org/wiki/FRB_180916.J0158%2B65