The explanation is that this is normal. Times can vary widely. As long as your percentages are increasing they should get done. If you can't finish it in two weeks you could encounter a timeout problem.

My slowest machine is a Pentium III/500, processor score 50. I ran a couple of recent longer than usual HPF workunits that took 10 days each.

My machine usually had an overall score in the low 130s with the UD agent and I once had a single WU run on it for about 5 days. I got about 2800 points for it. As has been pointed out, what you are seeing is likely perfectly normal. You just have one of the rare but occasional large WUs. One thing to keep in mind though is what else your machine may be doing in the meantime. 5% in 16 hours on an otherwise idle machine is not the same as one that is doing some other CPU intensive work.