If the estimated 1RM is lower than the previous training session, then that means that either (1) muscle damage is still being repaired, or that (2) the athlete is in a state of greater psychological stress than at the point of the previous workout. In both cases, that means there is a risk that central fatigue is present (but it may not be). Training while central fatigue is present will not produce optimal results. I have just written a detailed article about this, and will publish it within the next couple of weeks.