All work is sent to at least 2 computers, which both have to return the results. These two results will be compared against each other, to see if they match.
If they match, both will get credit and that's that.
If they do not match, the task is sent out to a third computer, which has to return it before it's checked against the original two results, and only when it matches will credit be given out.
If still not a match, the task is sent out again to a 4th computer, etc.
Since there are about 200,000 computers attached to Seti, with varying amounts of work in cache, and validation matches not happening all the time every time, it can take quite a while for your tasks to be credited. Especially when you're matched against someone with a huge cache (thousands of tasks).
Be patient, eventually it'll all work out.
Ancient Astronaut Theorists suggest that in many ways, you can be considered an alien conspiracy!