Is it because of the presence of R?
Vowels with /r/ following them are treated differently in English.
Most Americans never put an /r/ after an /ʌ/. Most English people don't put an /r/ after an /ʌ/ when they're in the same syllable (in fact, many of them don't put an /r/ after any vowel in the same syllable). In these cases, a word which historically should have been pronounced /ʌr/ is instead pronounced with /ɝ:/ or /ɜ:/. In Scotland and Ireland, they can and do pronounce /ʌr/, although not being Scottish I have no idea which vowel they use for slurp.
The history of the pronunciation of 'r' after a vowel in English is quite complicated, and when learning English you just have to learn that curd, herd, bird, all rhyme (except in Scotland and Ireland).