[robocat]

Listen carefully to different English accents, or even better try and mimic them.

There's a massive variety of vowel sounds in English: Sydney, Irish, Boston, Indian, etcetera.

English speakers can often hear the differences, and many people can produce the different vowels when mimicking the accents (country, city, person, foreigner).

[alimw]

I did not deny the fact that there is a greater variety of vowel sounds available in English. I merely doubt its explanatory power for the phenomenon you describe. But perhaps I am confused about exactly what that phenomenon is.