We can crunch the numbers any way we want, it's not like we can draw any real conclusions.
First, there isn't enough data to actually do any (valid) statistical analysis.
Second -- and much more important -- Jim isn't delivering his manuscript to some predictable, homogenous, PerfectEditingEngine. The publisher won't slurp up the draft and PEE out a hardcover in 4.61829 months.
It's a bunch of PEOPLE. What other projects have they got under way, because they cannot just put the whole business on-hold waiting for the next DF novel; not even a guaranteed hit is worth that! Maybe his editor is going through a divorce of their own, or moving, or just lost a pet, or is undergoing gender-reassignment surgery. I dunno, what other reasons has Jim given -- valid, understandable reasons -- for being so much slower to complete this draft? Maybe Jim's editor isn't playing "Keeping up with the Butchers," maybe they aren't hitting EVERY hardship Jim hit en route to finishing this draft. Maybe they've got it worse. We have no real way of knowing. (n.b. I don't remember all of Jim's (very good!) reasons; I may have forgotten one or two, or added-in one or two that didn't actually happen to him).