Wait_us() accuracy on stm32f767zi

Hello. Is it normal that code like this:
now = us_ticker_read();
time = us_ticker_read() - now;
sometimes gives value of “time” equal to 251 instead 250? I have tried creating new thread with osPriorityRealtime7 which seemed to do better than main function however every ~300000 loop the value of the “time” variable is not equal to 250.