It can sometimes take a week or longer for a work unit to be validated. Let's suppose the same work unit is sent out to two computers. Your computer finishes the work unit and sends it in. The second computer may have gotten an error, in which case the same work unit has to be sent out to another computer. That's how the validation process works.

BOINC uses a quorum system to validate work units. Each project has a different quorum size. Let's suppose it's a quorum of three, meaning three identical work units are sent out to computers. When they are returned, the validation program examines all three. If the results for all three work units look the same, then all three computers will get credit. But if one result looks a lot different than the other two, it is called invalid and thrown out. If all three work units look different, then the results are called "inconclusive" and three more identical work units are sent out to computers.