Why do I have at least 20 tasks waiting for validation going back to January of this year?

With your computer(s) hidden, it's difficult to say exactly. But Seti sends out two tasks for every workunit, to separate unique computers. Both of these computers must return a result before validation is started. If one computer for some reason doesn't return the work in time, it has to be sent out to a third computer, then wait for that one to return a result. If that doesn't happen... etc.
The returned result must also be the same between the two computers, else a third task is sent out to another computer as well, to check if everything is all right with the task and the previous returned results. Here you'll have to wait until that third computer returns a result as well before validation happens.

With tasks having a deadline of up to two months, it can therefore happen that work lost in transition, or by a computer out there, or by results not being the same the first time around, stalls the validation process. In the end though it'll all validate. You just have to be patient.

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Thank you for the very clear explanation!!!
I totally understand now!!!

Thanks again!!!!