The imported goods may be sold from one trader to another, and eventually exported. When this happens, the exporter can claim back from the government the whole of the VAT that should have been paid on the goods (as exports are zero-rated). However, if there is a "missing trader" further back in the chain of sales, part of this VAT was never paid in the first place. Hence, there is a loss to the government.
This can repeat many times, with the goods going round in a 'carousel'.