Has anyone noted this phonetic variation in /ə/?
In a comment John Lawler wrote:
Yes. Schwa is the only central vowel phoneme in English. In American dialects, it's lowered to [ʌ] when stressed, and its allophones vary all over the central vowel space when not stressed. That's because there are no other central vowel phonemes that it contrasts with, so individual variation is normal and unremarkable. Front and back vowels contrast strongly in English (2 high front, 2 mid front, 1 low front, 1 low back, two mid back, two high back), but there's only the one central vowel.