What is the purpose of uniform rate of application of water in watertightness test of water retaining structures?

In watertightness test of water retaining structures, it normally requires the filling of water at a uniform rate or letting the water pool stand alone for some time before actual measurement is carried out. The reason for such provision in watertightness test is to allow sufficient time for water absorption to take place in concrete. After allowing sufficient time for water absorption to occur, the subsequent measurement of a fall in water level is deemed to be caused by water leakage instead of water absorption
provided that adjustment has been made to other external effects such as evaporation.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.