This is actually one of the reasons some people will say English is sometimes complicated due to some certain rules and judgements involved in the formation of words and where they actually belong.
There is a popular rule in English language which everyone is surely familiar with which is the use of "an" anytime the next sentence has a starting word of a vowel. A good example is "an aeroplane"and "an egg". It is observed that the starting letter of both words are vowels (a, e, I, o, u) and this applies to all other vowel words that possess vowel sounds.
In the case of "user", an exception is applied to some special words which sounds like having a consonant starting with such words. This is sometimes why transcribing is important in English Language as it indicates the nature of the letters involved. A preceding word attached to words like user should be "a" and not "an". The phonological expression becomes more lovely when using "a" unlike "an".
Thanks for reading and I hope this helps.
According to the grammar rules, "An" is used before all words whose first letters are vowels. For instance, an umbrella. However, there are few exceptions:
When the "u" conveys a similar sound that the "y" in you makes for example union, or when 'E' makes a similar sound as that of 'y' in you,for instance, European. Then, the "a" is used instead of "an".
There are two types of words beginning with the letter "u" in English - those that begin with the sound similar to "ah" ("umbrella", "underpants", ...) and those that begin with "you" ("user", "unicorn", "university", ...). The ones beginning with "ah" would get the article "an", while the ones beginning with "you" get the article "a".