The truth: In many cases, fraud is claimed when mail comes back from the given address undelivered or undeliverable. But that's usually because the person has moved away, or the mail was misdirected or the address contained a typo or was garbled in the data-entry process. In others, addresses turn out to be vacant lots, storage units or government buildings, which usually turn out to be, again, the result of error, or are in fact legitimate addresses. Occasionally, suspicions are raised when multiple voters turn out to have registered at a single address. But usually the reason that they all live at a single address.
Typical case: In New Hampshire, a citizen became concerned because 88 individuals had registered with residences on property owned by Daniel Webster College. Further investigation revealed that all 88 were DW students living on college property.