Why does /ɪ/ often sound like /ə/?

To my ears, neither the British nor American speaker have reduced the vowel in the second syllable in seminar to a schwa. In the pronunciation of adequate on the same website, however, I hear the American speaker reducing the vowel in the second syllable to a schwa, while the British speaker even lengthens it a bit. The IPA symbol is identical for both.

Our ears have accents as well, and we often hear others pronounce words as we do even when they in fact do not. If your particular dialect is one where the weak vowel merger has taken place, then the difference you hear may only seem like insignificant positional variants rather than distinct vowels.