No, in this case it's real creep, not rounding error.  The task might miss some time and then when it runs xTaskGetTickCount() is 14 ms ahead (when the threshold is 10).  Then the next cycle is only going to wait 6 instead of 10.

Honestly though since you are accumulating samples and have a sample count and timed by the accel queue this code probably should just be dropped and just process the data whenever there is enough.  If you want to do anything flush the buffer and reset on line 267.