it's from both ways.
i was a vac student. i am about 80% sure I paid no tax. Therefore my employer paid no tax (because the annualised rate was below the treshold).
from a tax man pov you have to see that he doesn't want a highly skilled person paid just under the annual threshold for one month's work. while my example may not be robust enough, i am trying to show the kind of thinking tax authorities apply.
therefore while the annualising bit is actually something i do seem to remember from back in the day; it also seem sto make sense to me from the general manner tax authorities work.
of course if there is actual practise that works otherwise or legislation i am more than willing to admit defeat.