As per this question, Thread.sleep is not necessarily guaranteed to sleep for the duration you specify: it may be shorter or longer.
If you read the documentation for Thread.sleep, you'll see there are no strong guarantees over the exact duration which will be slept. It specifically states the duration is
subject to the precision and accuracy of system timers and schedulers
which is (intentionally) vague but hints that the duration should not be relied upon too heavily.
The granularity of possible sleep durations on a particular operating system is determined by the thread scheduler's interrupt period.
In Windows, the scheduler's interrupt period is normally around 10 or 15 milliseconds (which I believe is dictated by the processor), but a higher period can be requested in software and the Hotspot JVM does so when it deems necessary
Source, emphasis mine
How can I practically guarantee that the sleep duration will be at least the value that I specify?
The best practical solution is to time the sleep and continue to sleep while there is time remaining:
public void sleepAtLeast(long millis) throws InterruptedException
{
long t0 = System.currentTimeMillis();
long millisLeft = millis;
while (millisLeft > 0) {
Thread.sleep(millisLeft);
long t1 = System.currentTimeMillis();
millisLeft = millis - (t1 - t0);
}
}
Code and info is from here