Question about Ryans video on you tube IELTS argument essay

[Mohamed Elamin]

Hello All,
As I was trying to memorize the first argument essay written by Ryan ' IELTS argument essay demonstration' I went through the first example which says ' A very important aspect of teaching is the ability of the teacher to shape their teaching style to the needs of their students.'
well, it was written the ability of the teachers to shape their teaching style to the needs of their students. Why wasn't it written as the ability of teachers to shape their ....

is it a mistake or does it has a grammatical explanation?

thank you