I find the phonemic symbols are different for the same word among dictionaries. Take the word "tuck" for example. In Oxford Learner's Dictionary, its /tʌk/ for both British English and North American English. However, in Kindle's dictionary which is The New Oxford American Dictionary, it's /tək/. It doesn't make sense to me that one word pronounces differently.
It doesn't necessarily represent a real difference in pronunciation. The different dictionaries use different conventions.
To understand the reasons for this, you should keep in mind the importance of stress to the pronunciation of vowel sounds in English.
The most standardized use of the symbol "ə" is to represent the "reduced" vowel that is found in unstressed syllables in words like suˈppose or ˈhammock.
For most speakers, this reduced vowel cannot occur in stressed syllables. But the "short u" sound, which can be written "ʌ", does occur in stressed syllables: it is found in words like ˈsupper and ˈmuck. The "short u" sound that is found in stressed syllables is phonetically similar to the reduced vowel that is found in unstressed syllables. Because of this similarity, and because the stress allows you to predict whether a syllable has the "short u" sound or the reduced vowel sound, it is somewhat common for transcriptions to use the single symbol "ə" to write both the stressed and unstressed vowel sounds. Furthermore, at least some speakers think of these as being the "same" vowel sound in some sense.
In transcription systems that write "tuck" as /tək/, the symbol /ʌ/ is not used at all. This may be considered helpful in that it means that there is one less special symbol for the reader to learn.
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