Some people pronounce it differently due to their accent or what they heard growing up. For instance, in Persian, one says "Ramazan" when talking and not "Ramadan" because it is how it is said in the language, just as in English people often say "Izlaam" not "Islaam". For things not involving Qur'an, it doesn't really matter.
However, when reciting Qur'an, it is good to try to pronounce the letters as correctly as possible. If a person is genuinely unable to pronounce a letter correctly, it is ok, but they should try.