Transformer Secondary Voltage

This is because a step down transformer is needed given the distribution transformer is connectioned on the primary side to the transmission system which is by design at a high voltage in order to reduce power (I^2R) losses in the transmission line. Also as consumers need low voltage for commercial, industrial, and residential uses, they are on the secondary side (there are also probably other step down transformers along the way before the consumer).

It is due to ferranti effect.

The primary voltage of a distribution transformer decreases due to increase in length of the conductor between step-up and distribution transformer.