I have seen this sentence in lots of websites and I'm really confusing about it:

a good assistant uses their skills to help the master focus on their skills

When A good assistant is singular, why it says their skills while their is plural. It's the same for master and their skills.

I'm wondering if it is a grammatical mistake which lots of people are doing their websites, or a type of exception which I'm not aware about it.

I'm now confused about the real meaning of this sentence.