This happens for a couple of reasons: 

First, as you approach your goal, your taper allows you to rest well and gain from the training you've done, thereby allowing you to be on top form for your event. 

Secondly, your event imposes a significant training load on your body. The Banister Equation, a derivative of which we use to predict your performance, handles this the same way as it would a large hit of training - follow it with rest and your body will respond with fitness.