Why is creep a major concern in balanced cantilever method?

In balanced cantilever method, the moment is balanced along the length of the piers. However, along the extended cantilevers only a part of negative bending moment is balanced by prestressing bending moment arising from normal force induced by prestressing. As such, it results in large deflections induced by concrete creep. These partly unbalanced permanent loads generate creep which produces unexpected and unevaluated hyperstatic effect. In fact, the compressive stresses are very high in lower slab while they are very low in upper slab of bridges.

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