LabVIEW wait function in a while loop delay time not accurate

Need Your Help,Thanks!
I put a wait(ms) function in a while loop, and the value is set to 10 ms. I put tick counts(ms) before and after the wait function to calculate the delay time.
I found that sometimes the delay time is larger than 10 ms, sometimes can be about 150 ms. how could that be possible!
The snapshot of the VI is attached, thanks!.