> (Also, a lesser consideration: there are some native speakers of English who can distinguish certain sounds in their pronunciation but cannot, or cannot easily, hear the difference between those sounds. I would guess that either they learnt to speak partly by watching people's lips or their hearing has deteriorated as they got older.)
It may also be down to the conceptual buckets into which we place sounds when learning languages, especially our native languages. For example, many English speakers would be surprised to learn that 'p' has two distinct pronunciations in English, aspirated (as in 'pin') and unaspirated (as in 'spin'). Similarly, the 'l' at the start of 'late' is not the same as that at the end of 'wool', the so-called 'dark l' sound. (Caveat: IIRC, some varieties of English do not make the latter distinction, and I wouldn't be surprised if some don't make the former one, either.)
A couple of personal anecdotes:
- I have a friend who pronounces the word "hook" with a long "oo" sound, whereas I pronounce it with a much shorter vowel (we have very different regional accents). When I pointed this out to him, he could not at first tell the difference between our pronunciations. It was only when I compared it to the difference between "pull" and "pool" -- which he did pronounce slightly differently -- that he grudgingly acknowledged the distinction.
- A Northern Irish colleague expressed incredulity at the southern English habit of pronouncing "poor", "paw" and "pour" the same, when in his part of the world they were, to his ears, said quite differently. When he spoke those words, I could hear that difference too -- but only just, and it took concentration.