The use of the indefinite articles (a and an) is often dependent on the pronunciation of the word, irrespective of the starting letter.

For example: the word university is pronounced starting with “you” and hence is preceded by a despite starting with a vowel, the similar logic is followed in case of the world ‘utensil’.

on the other hand, words like honour and hour, have a silent ‘h’ and the pronunciation, starts with a vowel. So ‘an’ is used for them.