I am not a phonetician, but I will give you my understanding of your query and maybe it will help (maybe not). I like to imagine the IPA symbols as coordinates on a map. Often the coordinates of the sounds we make in speech are not represented exactly by the coordinates of one of the IPA symbols, but we use the nearest IPA symbol to represent the sound we are making.
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pronunciationkarenJust a quick addition. The one place where I might pronounce an 'er' ending /?:/ is when using a comparative adjective where I particularly wanted to stress the comparison, in which case I would transcribe it as /?:/So how would you pronounce the er endings?