Since you hold that the abbreviated letters are the names of the Sūrahs in which they occur then why are some of the sūrahs are also called by other names?
The abbreviated letters are no doubt the name of the respective Surahs but that does not entail that once named by the Almighty these Surahs could not be called by any other name. Sometimes the Prophet (sws) and the Companions gave these Surahs their own names too. That is why Surah Al-Baqarah (2) is called such in spite of being named by the Almighty. The companion too sometimes referred to Surahs by different names.