Translation into English of “交通银行”, “交通大学” and “交通职业技术学院” Obviously, "交通" means transport or transportation in English. But in reality, there are various versions of the translation.

"交通银行" = Bank of Communications (One of the key five State-owned commercial banks of China).

"上海交通大学" = Shanghai Jiaotong University (One of the best colleges in China, offering a large number of degree majors besides those concerning transportation).

"广西交通职业技术学院" = Guangxi Vocational and Technical College of Communications (One of the many such colleges co-sponsored by local education authorities and local transportation authorities in China, with three-year diploma programs offered but no degree).

Why are they so different? Is there any principle for translation or nomenclature?

By the way, China's national transportation authorities 交通部 now changed into 交通运输部. Its name in English was "Ministry of Communications", but now it is "Ministry of Transport".
For the universities, only 

上海交通大学 (Shanghai Jiaotong Univ)

西安交通大学 (Xi'an Jiaotong Univ)

北京交通大学 (Beijing Jiaotong Univ)

西南交通大学 (Southwest Jiaotong Univ, in Sichuan)

国立交通大学 (National Chiao Tung University, in Taiwan)

use pinyin for 交通 because of historical reasons. They all originated from one university, the old 交通大学, where 交通 here comes from an old literature and doesn't simply mean transportation.

Other "交通" universities will use the English translation, either transportation or communication.