Quellensteuer after job change (in other canton) but no address change

Hi all, I have spent some time looking through the various Quellensteuer threads but am not yet completely clear on what I need to do now in my situation - please excuse my dumbness on such things.

I am on a B-permit paying Quellensteuer. Previously I was living and working in Zurich, but have recently changed for a job in Canton Sank Gallen, commuting each day back to Zurich.

I was always under the impression that the Quellensteuer tariff was based on where you live, so I didn't expect to see a change in the amount deducted from my monthly pay. But it has changed and it says 15% in the first letter (from 9%).

I have seen the letter and I will ask tomorrow, but perhaps you could give me some information before asking.

I hope it's just an error.

Cheers.