We assume and expect consistent productivity from ourselves. Anecdotally, In my 20 yrs experience, I've never heard a software engineer saying to their manager "add 2 more weeks on top because my mental state is not at its sharpest these days". No SE in their right mind would put themselves in such a vulnerable situation especially when no one else talks about it.
The delays we naively anticipate are the obvious technical related ones and gotchas / cases we might not have considered for the problem.
This is a problem that is prominent in this field and unless I've been missing out on discussions, I've never seen it discussed.
There is a huge productivity difference between people that wake up in the zone vs any other zone. And I've yet to see an SE or any other human that can wake up in the zone every single day. It's impossible. Although this is something we can improve at an individual level by focusing on lowering the std deviation which will help us estimate better and imo it's one of the key factors why some SEs can give more accurate estimates than others assuming they have similar technical skillset.
Each SE deals with this fluctuation / "problem" alone. I've often seen SEs jumping on the management track when the gap between "being in the zone" start to consistently widen. Often after a burnout that never fully recovered.
You wouldn't see this being a problem in careers like HR/Management/Sales. Because an individual can still be somehow productive without being in the zone. But in software engineering, art, writing and the like, it's kind of binary and is very visible.