Such a condition may arise in case of a mosaic or mosaicism, which denotes presseve of two or more populations of cells with different genotypes in one individual.
it can result from various mechanisms including non-disjunction, anaphase lagging and endoreplication. It may also result from a mutation during development which is propagated to only a subset of the adult cells.
In this case, cells with 16 chromosomes could have arisen from cells with 32 chromosomes.