Well, it has some sth to with Chinese. This kind of situation only happens in korean words which came from Chinese and only happens in S.Korea. there's the rule.
the family name "이" is from Chinese 李,whoes pronunciation is "li" in Chinese, when the word's Chinese pronunciation begins wth "L"or "N" it may change as following:
1. if the vowel in the world (Chinese) come with i in Korean, you should delet the consonant "L"or"N". eg: 리 (李 Chinses pinyin:Li)~이 , 리유(理由pinyin: Li you)~이유 녀자(女子pinyin: Nǚ zi)~여자 례절(禮節pinyin: Li jie)~~예절...etc. But you can only delet the consonant when it is in the beginning , if not, then keep. eg 처녀(處女:chu nu)~~처녀(do not change, though it's the same 女).
2. the 2nd one is a little dificult since you may not know Chinese. But it's all right that you remenber all wrods without knowing why this way. even Many Koreans dont know it originally.
All this rules are nothing in N.Korea and Chinse Korean. Acctually, there ara nearly 70% Korean words from the Chinese language or Japan's Chinese words. So if you want to know exactly the words , you must know the Chinese character bihind the Korean words.
it must be some hard to understand, since my English isn't good enough...sorry...