A few points:

The true up happens when you file a return or if you request a Quellensteuer review.

The rates applied are average (kantonal) rates, this means that if you file at the end of the year you are either entitled to a refund (in case of a gemeinde with a low tax base/multiplier) or you have to pay additional tax (mirror situation).

In this way the additional monthly one offs, like bonus payments etc, are leveled out in terms of your tax rate.

A complicating factor may be assets held outside CH, although not always taxable in CH (real estate for instance) they have an impact on the determination of your tax rate.

All in all, a nice puzzle....