You tax rate is 5%. But still, your employee has the option of collecting from nothing up to 18% and they’ll be acting legally if doing so.
Whatever the difference, it will be corrected when reporting your taxes in the following year, either to give you money or to take it from you.
You may not like it (I certainly don’t) but that’s how it is.
Incidentally, I had the same issue with my employer. They were withholding 18% from my monthly salary. I was able to convince them to stop, after showing them that my ARC will not expire in the current year. Otherwise, they’d withhold 18% until after I renew my ARC. That’s the company’s policy, they said.