Why "they" is used instead of a singular object?

[kwfine]

Hi Alan,

Could you explain to me why "they" is used in the following sentence please?
I don't think the word "they" is inconsistent with "someone" in the sentence.

if a piece of information goes over someone's head, [they] do not understand it.

Thank you.

Kitty

[Alan]

It is normal informal usage for an indefinite pronoun. Formal usage is 'his or her'.