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Actually it isn't true at all, i have seen transformer's with whole range of KVA's. But the transmission lines have voltages in multiple of 11 because there is a need for power factor correction, and thus this could also be the answer to your question. See power factor of realistic power system lies somewhere between 0.85 to 0.9, and thus a multiple of 11 rounds of the resulting voltage to near about 1, thus increasing the efficiency of the system.
Its not standard or there is no specific technical background of adapting these voltage levels. 13.8 kV voltage is also being used in Middle East, USA, and some other places like Philippines. However, Its just a randomly developed criteria by the initial Electrical Generation, Transmission & Distribution (Utility) Companies in early last century. They adapted these levels and were carried on by the other suppliers/competitors in the Market. That is how it is now being used as benchmark.
If you find any other Technical description, then must share. Regards,
