light 87, you constantly use 'IPA'. This is actually simply an abbreviation of 'International Phonetic Alphabet'. The symbols in this alphabet, when used by a trained phonetician represent one, and only one sound. If that sound is pronounced in a slightly different way, that change will be indicated by subscript and superscript symbols. Phonetic transncriptions are enclosed within square brackets - [ . ].
Most dictionaries give phonemic, not phonetic, transcriptions, which do not record one precise pronunciation, but rather a broad 'average' pronunciation. Phonemic transcriptions are enclosed within slashes - /./. You can know what the dictionary compilers mean by the symbols they use only by reading the notes in each dictionary.
In the word 'mortal'. the sound represented by the letters or is a pure vowel in most varieties of BrE, but has some degre of /r/ colour in many AmE varieties. The symbols actually used, and the degree of /r/-colouring will depend on the phonetics editor of the dictionary you are using.