If I try to imagine someone saying 'claustrophobic" in a Midwest accent it sounds a bit like 'ah'. I can't get my head around 'plahsible' though.
Important point to bear in mind here is that we don't actually all hear the same sounds as each other. Depending on the languages and dialects to which you're exposed, especially when growing up, your brain learns to distinguish and group different sounds. So it's entirely possible for me and you to hear two distinct vowels where someone from across the Atlantic can hear only one, and vice versa. My girlfriend finds it hilarious that I struggle to distinguish 'ou' and 'u' in standard French.
Further complicating things is that, not only do we hear different sounds, but we label them differently as well. The way that I would pronounce the vowel in 'hot' or 'top' would, for me, define what a short 'o' sounds like, but to American ears it apparently sounds more like an 'a'. Which is funny because the standard American pronunciation of that vowel sounds closer to an 'a' to me.