The voltage output of a generator is set by design and system requirements. Nothing is special about 13.7 KV. In fact, most generators that I am familiar with in utility power applications are 24 KV, with the voltage being stepped up with a transformer to transmission level voltages.
Perhaps the question refers to distribution voltages. One standard is 13.2 KV, with the transmission voltage being 69 KV, 138 KV, or 345 KV. Again, it depends on system requirements.