This is fine. The varying pressures on the belt and temperature variation cause the pointer to move a little. If the pointer is in the middle after you tighten its nut with the timing belt at TDC it's correct. This is why it's supposed to be tightened at room temp and not on a hot engine. Even still, you've got to wonder about tension if it's done in the winter vs. the summer. Also make sure the tensioner tab is in its slot because this is something that many people overlook.