I don't really know enough about how the system works to give you an answer to the validation question, but I can tell you that it is not unusual for there to be periods with no work units available. Again, I know very little about how/why the project operates the way it does, but work units are generally released in large batches, and then the system waits a bit before putting out new tasks. I would assume there is some reason for this, perhaps it is somehow using previous results to determine what tasks to send out next or something.

But yeah, long story short, it is not unusual for there to be short periods for which no work is available. Usually it doesn't last more than a day or a couple days.

What I've realized is that just because it says "No Work Units Available" that just means that at that instance any work units that become available are quickly sent to clients requesting them.
Work Units Available is a "Buffer" and if all the work units available are being sent away to waiting clients as soon as they are generated, then the "Buffer" will show 0 available in the buffer. How quickly the work units are generated and made available and depending on the number of requests for work units are being made determines how many work units are available in the buffer.

Oh, that makes a lot of sense. What about the OP question about how the number of results awaiting validation doesn't match the number awaiting validation on the client end?