Radioisotopic dating of detrital minerals in sedimentary rocks can constrain sediment sources (provenance), elucidate episodes and rates of ancient orogenesis, and give information on paleogeography and sediment-dispersal patterns.
Why is it that different locations within a zircon have differing ratios of Pb/U?
Certainly it doesn’t make sense to say different zones within a tiny crystal formed at vastly different times.
Pb found in zircon is not common Pb from magma, but radiogenic Pb produced within the crystal after it formed.
So to answer the question posed in the title, we should not assume that there had been different amounts of common Pb incorporated during crystallization.
Th activity was measured to be varying from 2.0 to 2.8.