Why are bored piles usually cast higher than the required final level?

It appears to be a common construction practice that bored piles are normally cast up to a piling platform much higher than the formation level of pile caps. Then, the excessive pile length has to be cropped and removed to the correct level for the subsequent construction of pile cap. The reasons of such practice are as follows:

(i) It is technically sound to employ such practice because laitance, impurities and poorly-compacted concrete should have migrated to the top of the piles. Therefore, additional length at the top of piles is constructed to accommodate these sub-standard concrete, which are subsequently removed and cropped to leave high quality concrete over the entire length of piles.

(ii) In actual site practice, the details of construction sequence and access arrangements have potential impact on the possibility of constructing piles to correct level without the need of cropping.

This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.