Q. What do these words and especially the vowels flagged within them have in common?
A. Əə Schwa!
That's the diminutive eh/uh sound that indicates a vowel but doesn't really specify which vowel. Indeed the list is chosen to indicate how versatile [and omnipresent] the phoneme is in ordinary spoke English. While there is some controversy about you say potahto, I say potayto only a really earnest non-native speaker will you catch saying potato rather than pətato.
I know this because I am currently on a Gretchen McColloch and Lauren Gawne jag ploughing through their two-hander podcasts at Lingthusiasm. McColloch is in Montreal and Gawne half a world away in Melbourne but they've been having monthly chats, for the last 4 years, about language. They are both native English speakers but share an interest in linguistics: where science drives its way of knowing deep into the heart of the Arts Block. I've downloaded a clatter of these dialogues to my smartphone as podcasts which I listen to while driving or doing the washing up. But I always have an open note-book close-by to take notes . . . because my event horizon is coming closer each year. With great courtesy, Team Lingthusiasm have also paid to have each episode tidied up and transcribed into text. I can therefore read them in bed [where screens are streng verboten in our house] and more easily search back for the key quip-or-fact.
Mais revenons à nos schwas: here's the podcast & transcript. The word originally came from Hebrew and into German and indeed used to have its own: schəwa. But language is nothing if not labile [prone to change] as well as labial [lippy] and in that word it was elided to zip. I'll have to stop talking about schwa now because I'll say something goofy/wrong and I've given you the link to follow up with people who actually know what they're talking about. One justification for retaining the idiosyncratic unphonetic spelinge of written English is to point to the origins and etymology of the words.
When I start my Human Physiology course each year, I give The Lads [as Pharmacy Technicians, they are almost all women] a pre-Quiz so that they have something against which to gauge all the things they'll know at the end of the course. About half the class routinely fails Q10 Appendix - Colon - Duodenum – Esophagus – Stomach. Please put in descending rather than alphabetical order. I know that none of you, Dear Readers, would fluff that, because you pay attention. But I bet that none of us reflect often, or at all at all, on what our tongue, teeth, palate, lips, lungs and larynx are doing when we speak. But doing so helps explain how languages change through time and distance. Jacob "Fairy Tales" Grimm noted that these changes were consistent rather than arbitrary as [pay attention to your mouth!] b in PIE proto-IndoEuropean changes to p and then to f [pyro - fire; pipe - fife;] and then back to b [frater - brother]. While c shifts to h [cardiac - heart; cornis - horn] etc etc. And remember P & Q Celts. Ordinary folk living quite isolated lives made subtle changes to the way they said things and were still understood; and some of those changes became the new normal. Now I hope you'll be more schwa-aware in future; I will. And I'll have more to say about Lingthusiasm, because it is rich. And a hat-tip to Dau.II who is diligently doing daily duolingo in French and told me about the podcasts.